In an attempt to remap the right alt key to my left alt key I had run the following:
xmodmap -e 'keysym Alt_L=Alt_R'
Now I cannot revert it back to it's original state. How can I undo this blunder?
Just log out and log in again. Alternatively I find that just unplugging my keyboard and plugging it back in resets the xmodmap i'm testing.
This works as long as you do not make any changes to your keymap that loads at boot.
To be on the save side, many people recommend that you make a backup configuration, like so:
You can then load back the original settings at any time: