All you can do is learn from this mistake. Dandelions are on of the most nutritious foods on the planet. They contain every essential amino acid, a full spectrum of vitamins and minerals, and aid in the cures for so many diseases. Do your research on this wonderplant if you don't believe me. Also the roots make an excellent root vegetable that's especially good for diabetics as it contains Inulin (not Insulin). Also, their roots go very deep into the ground, and draw nutrients up from the earth below.
When you sprayed an herbicide on your lawn, understand that you spread a HERB killing concoction all over your property. Now your land is tainted with chemicals that will effect everything that grows there for many years to come. Common sense: don't spread poison, and plants will live. Another tip I can pass on is to plant clover all over your lawn. It is very pretty, attracts bees and other pollinators, it draws nitrogen into your soil and makes for more fertile land, and they choke out other weeds that take over the lawn. Yet again, the clover is ALL EDIBLE and super nutritious. The flowers taste just like sugar peas!! So in short, learn about the beneficial "weeds" and use them to your advantage. Who says a luscious lawn of red and white clover doesn't look as attractive as a nicely maintained grass lawn? I for one vote for a healthy, diverse, all edible, nature attracting lawn of clover over a nutrient drained grass lawn any day. The choice is yours. Learn how to work with nature, not against it :)