Hi! Our tomato seedlings were growing like the bees knees until this weekend when, over the last three days, they've all closed their leaves and fallen over. No sign of brownness on the stem so I didn't think it was damping off. What can we do??

I believe damping off occurs as the seeds emerge from the soil.
More likely they are not getting enough water. That could cause overnight fainting.

I have seedling under lights right now and I recently had to start watering every day. I have Salvia, tomatoes, cucumber and zinnias.

Thanks Bill! That's interesting - I water them quite a lot so I had wondered if I had over watered them! I gave them a drink this morning, so we'll see if they sort themselves out. Otherwise I think I might have to give them a dry spell...

I also wondered if they were 'leggy', but given they had been holding themselves up okay previously I didn't think that was posible.

Well good luck.
You did not say anything about water in your initial post but that sounds fine.
Chances are it is either due to water or a light problem because young seedlings do not require much else!

When that happens in my garden, the first thing I look for is "cut worms". They are about + - 1" long and live in the ground. They cut the plants off or nearly off at about ground level.

Was the growing medium wet, slightly moist, or dry to the touch when they were falling over?
Wilting can be lack of water, sure, but you'll know it because the media is dry to the touch, for several inches down. But it can also be a sign of compromised roots, such as root-rot, caused by fungus (and usually wet conditions.) This can happen if you water and let water remain in the tray underneath your pots or flats, or poor drainage.
The roots will be brown and not the usual bright white color.
I've found tomato seedlings pretty resilient in dry medium if the wilt is caught quickly and the seedlings watered.
I seldom have problems with dampening-off. Keep your germinating containers clean and use a good seed starting medium.

Turns out Big Bill was right - and you Kenisaac. I gave them two bursts of water instead of one yesterday and they have recovered. I think one of them may never be its old self, but the rest look a lot better!

I wish for you lots of sweet tomatoes!

Turns out Big Bill was right - and you Kenisaac. I gave them two bursts of water instead of one yesterday and they have recovered. I think one of them may never be its old self, but the rest look a lot better!