It depends on what block it is. If it's a 9-patch, for instance, where every piece is the same size to begin with, you'll just have a smaller block. If it's something more complicated, you may have to cut some pieces over again. To take just a simple example, let's say you are doing a 4-patch that should finish at 12". It consists of two squares cut 6.5" (finish at 6") alternating with four patches. The four patch squares should be cut at 3.5".
Instead, though, you have two squares cut 6", which will now finish at 5.5". You four patch squares were cut 3", so they will now finish at 2.5" each, resulting in a square that's 5" finished. That won't match up with your other two squares, so you'll have to cut them smaller.
I'd suggest making one block from the patches you have cut and see how it goes together. Then you'll know for sure whether it's going to work or not.