In an ever-so-slightly related event, I just discovered that on my setup (2-4 mon, depending how I'm feeling ) if I have a panel selected that is not nested on the same screen as my menu bar (regardless of primary, secondary, etc system designation; & floating in same window is not good enough), then the alt key does not function properly (e.g. doesn't activate title bar menu; can't alt+f, s for File>Save, etc).
Hopefully, I stated that clearly. Not a big deal for me and my workflow, but wanted to throw that info out in case it might help someone else.
Good luck to you.
edit: Now that I think about it, a couple of times I've been unable to playback footage using the spacebar or any other means. Fortunately, restarting my system is all it took in both cases.
[New question branched]
CTRL+S will get you there from any pane.
I stretched Premiere Pro over both of my monitors like I do now and then when editing complicated piece, and I selected different panels on the monitor without the menu bar. The Alt key continued to work. Perhaps it is a graphic driver? Or the way your monitors are set up?