If you've got a careful hand and some superglue you may be able to "reconstruct" the part that broke off, but it probably won't ever feel the same.
If the alt key was the one you used, I say take the other one off (carefully!) and place it where the other one was. That is assuming they are the same size.
If the alt key was the one you used, I say take the other one off (carefully!) and place it where the other one was. That is assuming they are the same size.