Hi Maud, have you noticed that your tortoise hasnt been eating for a while. Tortoises need to starve for about a month before they go into hibernation otherwise there is a risk of the food rotting in their stomach and maybe killing them. Also how old is your tortoise, it is reccommended that young tortoises are kept awake through the winter as their bodies are not yet strong enough to survive hibernation.
If your tortoise has been eating up until now then it might be worth making sure it is warm and not too cold as this will make your tortoise very drousy.