Gordon - sounds like either damping off, or they have gone leggy. If it's damping off, the seedling will have 'wilted', and the bottom of the stem might be going thin. If you think some are still OK, water them with cheshunt compound to help combat the damping off. If the seedlings are leggy, they will be totally healthy apart from the fact that they are drawing towards the light. If you have a coldframe or greenhouse, put them in there and they should soon upright themselves. I hope this advice is sound! Thanks :)
Pete
