It’s not common at all for a cockatiel to be on his back, unless there is something wrong or something caused it. And he should certainly be able to easily flip back over. Was this in the cage or do you let him fly loose? If he flies loose, he may have flown into something and stunned himself. You may have walked in just as he was coming to. A pet bird should never be left out of his cage unattended because this type thing can happen. If he was in the cage, is it a really large cage? Are his wings trimmed? Is he a very young bird? They are clumsy and maybe he fell. Is there something he could have been playing on and fallen off of? If you can’t see any possible cause for this happening, then you might want to take him to an avian vet for a check up. Some birds do have seizures and that is a possibility. If his wings are clipped, it might make it a little more difficult for him to turn over, but he should still be able to do it because they push up with the top part of the wings. Hopefully you can figure out what caused this. I would worry if he got stuck that way for a long period because of the stress and panic, and if it went on for too long he could die.