If you buy foundation, you are going to have to discard the comb which fell out. Are you sure that you want to do that?
If it cracks, the bees will manage, as long is it is still aligned with the frame. If it isn’t, they may get creative, and then you could have the same problem that you already have.
Why not rubber band the piece that fell out back into your existing frame? It isn’t that hard to do. 