You have gotten the answers already, but I will just elaborate a little.
Yes, some of your seedlings have "damped off." That is a fungal disease that little seedlings are vulnerable to, especially in conditions of too much moisture and too little air circulation. The very bottom picture, the little seedling that is so leaned over, second up from the bottom of the cells, second over from the left, is a classic picture of damping off. You can even see where the stem has thinned a little where it goes in to the soil. Here's a picture:
There is no cure for this and seedlings that look like this will die. It can't be treated, but it can be prevented. Prevention is about the right kind of potting mix that isn't too heavy and moisture holding and not over-watering. Watering little seedlings is tricky, because they have very small root systems, so they can't be allowed to dry out, but they are sensitive to over watering. Watering from the bottom helps - put your little cells in a tray and just put water in the tray. Only put enough water to just touch the bottom of the cells. Then the soil can wick up water as it needs it. Good air circulation helps and it helps to put just a pinch of cinnamon in a pitcher of water to water with. Cinnamon is a natural anti-fungal.
AND applestar is right that your seedlings are way too spindly. The spindliness, being very tall and skinny, with a lot of stem space without leaves, is a sign of being as she said starved for light. Lighting for indoor seedlings needs to be fluorescent tubes, just a couple inches above the seedlings (hung so they can be raised as the plants grow) and on 16 hours a day.
This was the seed starting operation I had for years (until my recent move):
Notice how close the lights are.
Healthy lettuce seedlings should not have any visible stem:
Honestly, your seedlings are not very far along and they are going to be difficult to save. I would start over, with a better set up and maybe better seed starting mix. Clean every thing in a dilute bleach solution first to get rid of any fungus.
Read the seed starting basics thread here: https://www.helpfulgardener.com/forum/vi ... 48&t=44183
With experience, seed starting isn't really hard, but there's some basic equipment (lighting!) needed and some things you need to know.
Best wishes for a lot more success the next time.