Well some plants that are planted upside down will succeed and send up top growth but somehow I don't think your tuberous begonias will, for the simple reason; it sounds like you have also buried them as well.

Begonia tubers ( and I am assuming they are from the information you have given) should be set with the soil just up to the shoulders of the tuber, or put another way, pressed into the soil until the tuber is flush with the top of the soil.

If you are lucky and they are not planted too deep developing top growth might push the tuber to the surface, and if so, you can gently lift them and plant them the right way up.

To be quite honest, but then again I don't know,because I have never done such a thing before, I am not sure what will happen.

Personally I don't think they will succeed but I hope I am wrong for your sake.

Sorry but that's my opinion for what it is worth. :(

I have seen them planted on their sides as they are concave and it is supposed to stop them rotting as water accumulates in the dip - where the shoots come from. If completely upside down then it is possible that they grow then move up towards the light, but best not to stress them that way. Am sure that I have planted some bulbs the wrong way round - Oxalis and Anemone can be difficult/impossible to find the top and bottom and they all came up. As Teegee has said if they are not too deeply planted then it is possible that the shoots will find the light.

Its often possible to see the dried up remains of last years roots on the underneath side. If you have only planted them recently I would take them out of the pot and have a look. They are often slow to start so you will probably be able to do it without damaging them. If one side is a bit more hollow than the other or if you can see very thin dried roots put it back the right way up but leave the top out of the compost as above. You have nothing to lose as if they are upside down it would be very surprising if they survive.

Sorry I didn't notice you had put them in the garden. Its usual to start them off indoors in pots in the warm. I would try hard to find them and start again, as you stand very little chance otherwise.