This is not the same as NVDA losing focus, and being fixed with an alt+tab. That is an entirely different problem.

This is NVDA losing the ability to recognize all gestures.
For example, if you are in firefox, an "h" will not take you to the next heading. If you press NVDA+Q to quit NVDA, it will not happen. If you press NVDA+N to open the NVDA menu, it will not open.

Alt+tab will still have its normal Windows application switching effect, but that is not reliant on NVDA, and it doesn't fix anything.

The most reasonable way to fix this problem, is to press the NVDA launch shortcut (usually Control+Alt+N), to restart NVDA.

Gene asked:

Are those having the problem using Windows 10?  I've never had it happen in Windows 7 or XP.

Yes, I have only experienced this in Windows 10 that I can recall. Many versions of Windows 10 however, and versions of NVDA going back at least as far as 2019.1 beta, and possibly earlier.

Luke