Have you been feeding and watering them?, If you have them in a greenhouse and its too warm they will do this.They will need watering every day if not every two days. You have to keep an eye on them daily to prevent this from happening

Like Hijuju says, the first thing to check is watering. If watering isn't a problem, and you are growing in the ground, or are using a soil based potting mix, it might get verticillium or fusarium wilt. If so, try again with wilt resistant varieties. If in pots, bin the potting mix--you don't want that in your garden!