How do I tell if my Garter Snake is hibernating?

Today I came home from school and he was tangled slightly in the corner of his cage. His tail was up on a branch like thing I had in his cage. I assumed he died and took him out of the cage. He was very stiff and he smelled like a dead animal. Not to be sick...but,he did. So,I think he died. My dad says all reptiles smell but,I don't think like that. The furnace in my house went out last night and was not working,so,it did get quite cold. I did not have the heating mat on last night due to my cell phone being charged. Can someone PLEASE help me?