Q.How Do I Keep Lemon Seedlings Alive?
My lemon seedlings were thriving, but suddenly they’re starting to turn brown. One has already died and I want to save the others. Currently they’re all in one pot and until I noticed the first plant going brown, I’d kept them on a windowsill that got morning sun with a gallon plastic bag over them to create a greenhouse effect.

Certified GKH Gardening Expert
Let go of the plastic bag. Those are good for germinating seeds, but once the seeds have germinated, the bag is no longer needed. Now they need good air circulation to prevent fungal disease. Make sure they aren't being overwatered, also.
https://www.gardeningknowhow.com/edible/fruits/lemons/propagating-lemon-seeds.htm