As a public service, I'm recording this issue and solution here, rather than a throwaway tweet.
I've noticed in the past couple of months that the shift, alt (or opton if you're an old school Mac user) and spacebar functionality within Adobe Illustrator had stopped working. Examples would include when I use the alt key to duplicate selected content whilst moving it, or when I hold the shift button down to move content at 90 or 45 degree angles to its original position. Or even the old 'move the artboard' trick by holding down the spacebar.
The behaviour was sporadic and when they weren't working, holding the keys down had no effect- even holding down the spacebar did not change the cursor to the grab-hand.
After much forum searching, and going through many suggested solutions, I spotted that people were claiming that quitting Firefox solved the problem. I wasn't using Firefox but I had started to use Chrome a whole lot more.
The solution was to quit both Chrome and Illustrator, but restart Illustrator first. There seems to be a conflict with key commands between the two and I think whichever app opens first gets precedence.
I hope this helps someone with similar issues, as it is so frustrating. It makes it very difficult to be productive with one metaphorical-hand, tied behind your back.