A tortoise falls quite often on its back. This usually happens when it crawls between rocks and stones.
However, it must get on its feet as soon as possible; otherwise it will die, baked in its own shell by the sun. So, it will thrash around with all four legs, trying to get a firm hold on the ground with its long claws.
If it is not successful, the tortoise will waggle its head rapidly. This usually turns it over a little, and then it will catch hold of a patch of dry grass with its back claws, and pull itself on to its stomach. So, if ever see a tortoise struggling to get on its feet, do give it a helping hand!