What type of bator do you have?? I have a LG still air and have been fighting mine to keep it right. I set it and had it right on the money for a week before I put my eggs in. It did great for about a week and now I have to check it about 30 times a day to make sure it is right and not to cold or hot.
Something like this usually happens at least once, (if you're lucky), in everybody's lifetime if you work with animals. I've made more mistakes than my fair share & some of those were my own fault. This wasn't yours - you had checked the situation out & everything was as it should be at the time. You just can't watch 24-7. It isn't practical. Don't beat yourself up & most importantly, don't give up! Replace the failing equipment and try again. I remember one time I thought my digital thermometer was reading 110, but the display was failing due to needing a new battery & the first zero was not showing up completely. I always keep two in there now -1 digital which also reads the humidity level & 1 old-fashioned mercury thermometer that the unit came with.
I hope your thermometer is just going bad & the babies are fine. Good luck to you.