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Keyboard commands are essential to using Adobe applications like Illustrator or Photoshop. Unfortunately, some of the most important, cross-application commands occasionally break —the ones that are the most ingrained, because they work the same way in Photoshop, Illustrator, and other apps. I’m looking at you, hold-the-space-bar-to-get-the-Hand-tool, and you command-and-space-to-Zoom-in, and yes you, command-option-space-to-Zoom-Out. They break randomly, with no explanation and no warning, and it’s as if someone pried the ‘e’ key off your typewriter: sure, you can write a novel), but it’s pretty inconvenient.
Every time these keyboard commands break I curse for a bit and then google around to try to find the solution. Support boards show similar complaints, but no definitive solutions. Wishful, cargo-culty suggestions abound including “reset your preferences”, “restart”, “quit Chrome”, “no—quite FireFox!”, some of which work occasionally, but none of which work for everyone.
It’s not the app that’s broken; it’s the document. Try creating a new document. (You may need to restart Illustrator or your whole machine).
I believe the keybindings break because the .ai file is corrupted. When I noticed that keyboard shortcuts were working on one file but not another, I junked the broken file (try reverting from a backup or copy-and-pasting your work into a new file) and was able to use my keyboard shortcuts again. I’m not sure how the file gets corrupted, but this is a start.