Tortoise Flipped Over, Eating?

I found my russian flipped over when i walked into my room one day. I don't know how long he had been flipped. I noticed that he has been eating less than he usually does and he isn't as dramatically active, he is still walking around a lot but not nearly as much as before. Also, he has not pooped since then and I gave him a bit of timothy hay today and i let him soak for a bit to get a drink hoping that will help. If nothing changes in the next few days I will call my vet, what else could be wrong with him? is this from flipping over?