As the title says, can a tortoise survive on their backs for a couple of hours?

I ask as last week I went out for 3 hours to a birthday party, when I got back I got a shock when I found George upside down not moving his legs or arms. Luckly he was not under the light otherwise he would have got too hot. Would this have harmed him in anyway? he is eating, but im just so scared he will do it again. I have removed most of is bark that he likes to climb on as im just worried if he over turned again it could kill him.