Tomato seedlings require good light warm temperatures (70F+) good air circulation..... the seedlings require gental handeling
without mor info on how YOU are growing these plants can only guess as to problem.

Yes, more details are necessary, especially about what you are growing them in, indoors or outdoors, how often you water, if you are using fertilizer or anything else related to your technique.

In general:
1) Use warm water for irrigation and once the seedling emerges, you don't want the soil to me constantly moist. Let the surface dry out or you can get damping off disease.

2) Use sterile soil and containers for growing, again to avoid damping off and other diseases.

3) Don't over-fertilize or you can 'burn' the seedlings. I never fertilize mine at all.