What temperature is her cage kept at? It wouldn't be a bad idea to keep it a degree or two warmer after this. Once they attempt once, they can attempt again easily in the couple weeks afterwards.
Getting a thermostat is a very good idea & should help eliminate most of the possible issues. And as Twobytwo said, light can be a factor. Make sure she has 12-14 hours of light during the day, not depending on natural light. It's usually easiest to put a lamp on or near the cage & hook it up to a timer so you don't have to worry about forgetting.
Make sure your heat lamp is a ceramic heat emitter & doesn't give off any light. And also make sure that it's the right strength to keep the whole cage warm. A lot of people try to heat just the sleeping area & that's not going to work. If their bed is warm & the rest of the cage cooler, they just won't leave their cozy bed. I can't blame them!
I found it helpful to have a thermometer that measures the high & low temps & can be reset. That allowed me to make sure the night temperatures were staying steady & to take note if the temperature was dipping too low, indicating an issue with the heating system.
Sometimes, you just can't prevent them. Accidents happen, such as heating system elements giving out, a lightbulb burning out, or sometimes hedgehogs become more temperature sensitive & the temp they've been fine at for months, they'll attempt hibernation at. Lily did that & had several hibernation attempts as I bumped the temp up & up until we settled on 78-81F. Not all hedgies need it that warm, but she would attempt hibernation if it dipped down to even 77.
Also watch for drafts - even in a warm cage, if there's a cool draft going through from a window or door, that can sometimes cause an attempt.
I don't want to make you paranoid though! Usually attempts aren't an issue as long as the heating system is set up properly & you have a light schedule.