If it was a fungus it was not the only reason for your poor lawn. There are other things far more important as to why your lawn looks like this. Unhealthy grasses are very susceptible to fungus, insects...disease.
I have to assume this is cool season grass. Where is it your live on this planet? Here is a check list of what I am worried about:
Not enough regular watering, deep regular watering.
Your grass is cut way way too short. This might be my first choice for your
No aeration once per year by taking plugs out of the soil?
Do you bag your clippings? No mulching mower have I seen can chop up grass clippings small enough. No benefit at all allowing clippings to stay on your lawn. None. Thatch for sure.
Fertilizer? What is your fertilizer program if you don't have one this will be monumental to change. Do you have dogs in your backyard?
My recommendation is to rent a sod cutter and remove that dead grass and thatch. Use the old sod for another bed in your landscape.
Rake, grade (water needs to drain somewhere), roll with a water filled roller, rake and grade and roll again.
Install fresh sod running parallel to your home or perpendicular to the slope of the lawn bed. If your home is drained correctly the two should mesh. Hire a grass seed spray company if you want to go cheaper. Seeding by hand...we can lead you through this is you want.
Water shallowly until the sod can no longer be lifted off the soil. Water shallowly and often to keep that soil best moist if seeding. Never allow it to dry out. Shallow watering often...during hot weather that could be 4 or 5 times per day.
When the sod is firmly attached to the soil it will be ready for its first mow. Mow NO shorter than 3 to 3 1/2". Do not mow right after watering when the soil bed is soft. Bag those clippings.
When your grass seed crop is about 4" long, though sparse, mow at 3 to 3 1/2 inches high. Bag your clippings.
Now is the time to fertilize, usually 5# per 1000 sq. ft for the Scott's and Home Depot stuff. Dr. Earth is 9# per 1000 sq. ft. I think...again, this is extended release, not immediate release, healthier for the grass, perfect formulation and you might get away with 2X per growth season not 4. Make sure your fall fertilizer is lower in nitrogen...that is usually the culprit for lawn fungus.
Next year, make sure you aerate by pulling plugs, not by using sharp pointy things on your shoes! Rent this machine anyone can run little effort. Best to share this with neighbors so they too aerate their lawns, increasing everyone's home value. Very inexpensive to rent.
Water ONLY when you see your footprints stay down on the grass after the first mow. Water deeply and I mean 4 to 6" below the surface. Do not water again until you step on your lawn and the blades of grass stay down to show your footprints clearly. Water deeply using the amount of time the last watering took to get moisture down 4 to 6". Do not water again until you see those foot prints.
In a few months or by next season your grass will be trained to have deep roots not dependent on shallow moisture. Serious savings in water if you aren't on a well. Even on a well with plenty of water, this practice makes healthy grass. Too much water is death and dollars.
How big is your lawn? Where do you live? Are you solidly in love with the size of your lawn? Check out this quick little video (by Lowes no less) on the BEST way to edge as lawn. The only way to edge a lawn. Maintain the radius until you need to curve the other way. the best way to edge any lawn don't worry about the straw stuff
Then fertilize your lawn with Dr. Earth Lawn Fertilizer, balanced, purchase the one meant for the season; spring, early summer, late summer and fall. Never throw by hand, use only a rotary spreader. Scott's has a great one handed cheapie I always use.
Dr. Earth's or similar (check the ingredients). More expensive, longer to green but you'll need 2 or 3 applications versus the normal immediate release synthetic formulas of at least 4 applications per year. "Organic" but balanced fertilizer with added bacteria and fungi. Can not go wrong. Good for the soil, great for your grasses.
Do not mow shorter than 3"...better is 3 1/2 inches. Need the top growth to support the root systems on cool season grasses, to be able to photosynthesize enough to make carbohydrates those grass species roots and food factories...your grass plants need to be vigorous.
Once your grass is established whether sod or seed, then you only water when the blades of grass you step on stay down. You see your footprints. If the blades of grass pop back up it is not time to water.
When you see your footprints, water deeply...get a shovel and chop into your lawn bed to see the depth of moisture. Watering deeply means at least 4 " deep even 6" deep. Allow that soil to dry and only water again when you see your footprints stay down. This trains your grasses to have DEEP roots.
Deep watering also inhibits weed seeds from germinating. Weeds are shallow rooted, train your grasses to have deep roots during the time it takes to dry out the weeds won't have enough moisture to survive.
The height of your grass crop at 3" - 3 1/2" shades weeds seeds from germinating.
This correct height also cools and slows evaporation of water.
Aeration by pulling plugs once per year allowing the plugs to disintegrate where they fall will produce even more roots, air and recycle the organics, and make those organics, already decomposed available to feed the soil organisms.
Sharp sharp sharp blades. Don't skimp.
No other applications should be applied or considered; no liming without a pH test, no fungicide, NO GRUB control. Not another thing should be applied without ummm, asking us first? No weed and feed! Perhaps spot weed sprays for broadleaf weeds but just doing these proper maintenance practices will stop any need for 'bandaids' to kill weeds.
Do you have a line trimmer? (weed wacker)? Do you have a blower? I recommend Stihl, gas powered not electric! Edges are what our eyes see once you've a sea of green. You might do better with larger plant beds and less lawn. A lawn with definite edges done with definite radius that don't change until reversing the curve...here is a sweet little video I found that shows what I do for all lawns, for decades, for rich people, for lots of money, that looks and feels professional...