I put down grass seed April 20th and covered it with straw I'm in zone 5b. Before I did it I checked the 10 day forecast and we were not supposed to get any temperatures below 40. Yesterday we got snow and the overnight temperature dropped to around 30-32. None of the seed has sprouted yet, do I need to redo? 
Zone 5 should work fine for sowing grass seed in April. It takes 11 days to germinate after sowing and then one should be having their first virgin mow. If it is cooler or wetter than normal the germination will be much slower...up to 3 weeks before your first mow.
Normally I would not mention fertilizer for baby plants but you've used straw and it is slowly decomposing as we type. Decomposition organisms need nitrogen to do their job properly and more quickly. I would lightly apply a high nitrogen fertilizer. This will get that straw decomposing. How much did you put down? How thickly?
How did you prepare your seed bed? Please tell me you rolled it with a water filled roller, at least once before you seeded?
If you have not, go rent a water roller for compacting a lawn seed bed and go over your lawn in all directions. Don't worry about the babies that you squish. There will be plenty more. What was the setting on your seed spreader, what kind of seed spreader, how many pounds per 1000 sq. ft.? Or did you throw seed by hand?
I am not happy with the seed...whenever a package of cool season grass seed says .1 percent? weed seed that reduces the credibility of that seed and company. But no matter, if you learn how to mow, water, aerate and fertilize correctly you will be able to deter pretty much all weeds.
Mow HIGH. No shorter than 3 inches and 3 1/2 inches is ideal. Sharp sharp blades. Bag your clippings and use them in your compost pile. They DO NOT fertilize or help a lawn in any way. Great way to get thatch built up for sure.
Water DEEPLY and allow the soil bed to dry out before watering again. This 'training' begins right after the first mow. Watering a little every day is the kiss of death of a great grass crop. The idea is to make your grasses' roots grow deep to get at the water as the surface water dries up. No water restrictions will cause your lawn to go brown.
Using a spade, water for X amount of time and slice into a part of your lawn's bed. Pull the soil back to see how deep the water you used was able to reach. After your first mow you will be watering more often and shallower. Most cool season lawn soils are fairly wet right now so this training won't work until it gets warmer. Just be aware that the deeper the water gets in the top soil 4 - 6" that is where the roots will grow to get at that water. Watering shallowly is fine for shallow rooted grasses that you find in warm season grass mixes.
Cool season grasses have huge root systems unlike the warm season grasses. This is a great thing because weeds are generally shallow rooted. By allowing the soil at the surface to dry out you are inhibiting germination of ANY seeds and weeds will not be able to compete with the grass for water. So way cool. Cool season grass lawn are very cool. Easy peasy to promote the vitality of your crop while discouraging weeds to germinate and get vigorous.
The reason you have to leave 3 to 3 1/2 inches of top growth is to FEED those roots. Plants make their own food. Fertilizer is not food. Without enough photosynthesizing top growth those roots will not be supported and fed properly by the plant.
The other cool thing that happens with 3 1/2" grass blades is that any seed on the soil will not be able to germinate and get vigorous without sun. The grass blades are shading the soil.
Also, the shading of the soil slows evaporation of water. Saving water. You will eventually train your grasses to only need 1" of water per week delivered in one watering. Humongous saving and efficient use of water.
Aerate pulling plugs of soil and sod out of the lawn to be left right where they fall! Once per year!
Fertilizer needs to be applied 4X per season using the proper formulation of NPK. Extended release is best, slow greening is best and healthiest for your lawn. Here is a tip; find Dr. Earth's Lawn Fertilizer! It is more expensive but big deal. You will only need 3 applications per season not 4 if that helps. That fertilizer is oh so ORGANIC meaning it is extended, slow release and it comes with bacteria (thatch eating bacteria) and fungi, mychorrhizal fungi...good fungus.
I am a gosh darn expert with decades of experience installing and maintaining cool season grasses and I tried this stuff on my own lawn and was blown away. Immediately cancelled all Scott's and other fertilizer orders to use just Dr. Earth. Oh my goodness, the difference was spectacular.
You follow these instructions and you will have a nice thick lawn.
Did you roll that seed bed before you seeded?
How much seed #'s per 1000 sq. ft. did you apply?
Did you use a simple rotary seed spreader? Or did you throw it by hand?
Have you applied fertilizer already and if so, what formulation and how much?
Be patient, give your seed a few more weeks. If you did not roll that seed bed as yet you most certainly have a window of time to do so. Please send some pictures. If you didn't roll we can fix that later, not as easily if you were to roll now but I'd need to SEE what you are seeing to help best.
Just leave it and wait. I wouldn't expect any germination till you get night time temperatures above 50 so you were probably too keen to get started anyway.
If nothing happens in 4 weeks, try again. Unless something eats the seed, a bit of cold weather won't do it any harm, even if it had started to germinate - it will just stop growing till it gets warmer.
(And even if you hadn't sown the seed yet, waiting 4 weeks after the last frost or snow is still a good idea - you can't rush nature into doing things as fast as you would like!)
Personally I would always sow grass seed in September, not spring, when the ground is still warm even if the air temperature drops lower than expected.
I don't know why everyone insists on rolling. If you leave the soil loose the roots will go in deeper easier. Just stay off the lawn until the roots have tightened up the soil. It's good to stay off it anyways. Plus, when you compact the soil then fill and compact again, then seed, the roots go into the dirt and take up space. Sometimes as much as inches of space. So you thought you got the level of the dirt where you wanted it only to learn that your lawn sits 2 inches over the level of the sidewalk... leave the soil loose, it will grow in better, germinate better, and the level of the soil will stay the same. The roots will firm up the soil and the water will compact the soil as the roots take up space. Don't ever compact soil. If you do get dips you can add soil in 1 inch amounts at a time after the grass is full grown.
