Is my lemon ricotta cake doomed without flour? I inc baking powder but forgot flour!!

That depends. If it's more like a cheesecake or a flourless cake where the texture comes mostly from the cheese and eggs, probably not. But if it's a regular cake with a decent amount of flour, then it's probably doomed.

Can you mix the flour into the wet ingredients or is it already baking? If it's already baking then maybe you've just made a very interesting ricotta custard or pudding?