What are your night-time temperatures? If it frosted, that could be the problem. What kind of light is that? If it's sunlight that isn't filtered by a window, plastic or such, that could be the problem. Are they in the same location as when you first planted them?

If you watered them with something not gentle enough, that could be the problem (but most of them should perk up again if so). If you use a hose, use the mist setting.

If you covered them up dark too long, that might be the issue. If you have to cover them, it's good to let light through, and provide some air, even if it's cold outside.

Where is this?

Did the soil dry out?

Did you overwater?

Did you fertilize them? That's usually a bad idea at this stage, unless you know it's a safe kind for seedlings (and even then you don't need to do it).

It's also possibly a cat slept on them or something.

Did they ever perk up?

When the seeds sprout, remove all but the strongest one from each cell. Clip the rest off with small scissors. Make sure they are in full son outside or close under grow lights inside. I'd throw these out and start over.