Hi,
is there any sign of fuzz on the dirt? Do the seedlings suddenly wilt and die? Is your soil very wet? These are things that all lead to damping off - a mold/fungus develops and wipes out seedlings.

There are a couple of things you can do today. Put a low fan on to circulate the air around your seedlings. Second do you have any cinnamon powder? It works as an antifungal naturally - just shake the cinnamon all over the seedling tray and leave the stuff on top of the soil. This might help. I can't see an image of your tray so I'm just guessing from experience.

Please let me know how things turn out.

Cheers,
Peggy