Damping off or stress?

If your seedling is drooping hard, the issue is usually either over or under watering or that the seedling has stretched too much and cannot hold itself up anymore. You can add a little more soil around it to support it better, and if it seems to want to stretch more than it should, it's a sign to add more light. However given this also happened right after taking dome off, consider if the air humidity change was too big and you should get the RH up in the tent, or if you just haven't factored in how much faster it loses water when it's not covered. You can also just try to put the dome on and see if that makes it go back to normal, that's a sign that it's environmental stress and you might want to just consider keeping it on a little longer especially if you can't up the tent RH.