You may have had damping off disease (several organisms can cause this) - which will not go away with sun. Don't use the soil for any other seedlings - why take the risk. You could sterilize the soil, but just use new. If you are not starting seedlings in soiless mix then do so - or definitely sterilize your soil (lots of ways to do this). Damping off is quite deadly and can easily kill all the seedlings in a flat if allowed to flourish. But you probably could use it for potted up plants as they are immune to damping off after they get past the 2 or 3 leaf stage and develop a thicker stem. I recycle my seed starting mix to my larger pots (into which I transplant larger seedlings) with no problems. Your question is quite vague though and I worry that whatever killed your plants may be lingering.