I had something very similar happen to my seedlings. Killed three of them and to this day I dunno if I over-watered, under-watered, or baked them by having a 125w CFL two inches from them, with the heat being 80 to 90 directly under the reflector. I did an autopsy afterwards and found the root just below the soil level to be shriveled, darkened, hard and tough...I believe thats a sign of damping-off which I think I caused by overwatering but I'm still not sure. I posted a drawing of it here: http://boards.cannabis.com/showthread.php?t=85579

Now I have started new seedlings (bag seed!! Not wasting my good hybrids till I can get past the seedling stage!! lol) and am doing controlled experimental watering, doing each one differently. I also have a new 465cfm vortex exhaust fan as well as a 265cfm Dayton intake fan. It's still warm here in Socal and 75 during the day but I am hoping the fans keep the temps in check.

I don't know if any of this has been helpful, but thats the extent of my indoor growing experience.