My original keys wont work after making the copy keys. But the copy keys work fine. How is this possible?

More than likely your original keys and lock (which we can consider as just the different size brass (usually) pins in the tumbler) these surfaces have worn down slightly over time. So, when you copied your original keys (depending on how the blank keys are cut, it could be a little different from the original). Now where the peaks and grooves of your original keys are worn down some, the peaks and grooves on your new keys are not worn down, and will have sharp edges if their not smoothed over some. Anyway, what likely happened is your new keys with their sharp edges/taller peaks, etc., cut into or wore down the tumblers in the lock after a few uses, and now your old "worn down" keys no longer work.