When goldfish flip over like that it could be as simple as "gas".  If your fish does this and then gets better quickly within a day or two and doesnt seem to be swollen looking or anything like that then it probably will be ok.  You SHOULD vary the diet regularly including from things like frozen blood worms, flake, etc in this case though.  Usually when they flip over totally it's more severe and could be related to infection or possibly genetic defects or improperly formed organs.

When they dont try to flip over by themselves it means the problem has been a long term issue and they are now exhausted from trying to turn back over.  In this case they are very stressed and will likely not survive unless intervention occurs.