Google "damping off." I suspect that is your problem.

If plants wilt and die, I would suspect damping off...too much water and not enough wind.

If they stunt and turn yellow, I would suspect something in the starting soil [been there].

I'm down in SC (zone 8b) and won't put my tomatoes outside until after April 15th which is our last frost day. If your plants freeze their cell walls lose structure and they will look permanently wilted. I would cover them during the night for the next couple weeks or whenever you hear it's gonna drop below 32

Use 1 tablespoon of Epsom salt to 1 gallon water when planting. No more starts will die. No transplant shock. I wouldn't plant without it.

All these plants are inside under lights in original containers.

They are on a heating pad and it a room that is 70-75 degrees

You should take them off the heat mat within a day or 2 of germination. Excess heat may have been a contributing factor to the apparent damping off.

You don't mention how big the plants were, what size the pots are, what type of potting medium you are using, you mention the frequency that you water but don't say how you are watering or how much water you are giving...

Do they need watered ever couple days? Have you picked the containers up and felt their weight prior to watering? Agree with the others that it sounds like damping off. It is caused by overwatering, poor air circulation, a non sterile seed starting mix, a mix that is too water retentive, and just in general it needs a subpar seed starting environment.

Also agree with Lonejack that in a 70 degree room there is no need for a heating mat once the seeds sprout.

Rodney

4" pots, plants were about 2" tall, potting soil, I watered every other day. I did turn the heating matt off.

Try letting the soil dry out between watering.

A 2 inch tall plant in a 4 inch pot shouldn't need watered every other day. I would say the probability of overwatering is high. Overwatering kills plants quicker than underwatering.

Rodney

What planting medium did you use? I seem to have run into some problem mixes.

I used potting soil and I DID NOT watered every day - other day.