Sounds like damping off, did it start in one section and move across the tray? Does it look as though they have wilted? If you have any healthy ones left then remove them straight away and re pot them into clean compost, try not to get any of the old compost in with the new. I don't think there is any way to save the affected ones.

It could to do with you spraying them and not being able to support the weight of the water.

This is a kind of mould that attacks the base of the stem and is sometimes caused by using pots that have not been thoroughly cleaned or it can come from the compost. If you can get hold of any cheshunt compound watering with that usually fixes it - a soution of plain copper sulphate might do if not. But certainly move the healthy ones asap and you might still save them.Its infuriating isn't it?

Damping off or you are just not getting enough water to the roots

if the plant stem just above the compost is black then its damping off,cause over crowding and poor hygiene,.The time to use cheshunt is at seed sowing its a prevention not a cure

Yes but if only half of them have died it often saves some of the rest. Overwatering can cause damping off as well. If you wet the compost properly before sowing you shouldn't need to water any more for a while and in any case don't water if the compost is not well on the way to opeing dry. Daily spraying is making the air too humid and encouraging fungal growth.