Microsoft Ergonomic Keyboard 4000 with one or more single keys not working.
Use a key mapping tool (macOS users: Keyboard Viewer) to ascertain that indeed there is really no signal coming from the keyboard rather than just the wrong one. If you do see a keycode when you press the key, you need to have a look what kind of other tools you might have installed that intercept the key. In the following, we assume you do not see a keycode.
Good one. I have seen two major suggestions: the contact layers may be misaligned, or there may be excess static in the keyboard.
Remove the non-working key. This is easiest when you press down the keys in front of it, slide a flat instrument (e.g. a knife blade) underneath the key, and then twist the instrument. Do you see debris underneath? Remove it. Does the key work now (press the plunger down with a pencil)?
If not, it's time to open the keyboard.
Open the keyboard according to this video: https://www.youtube.com/watch?v=ANfPuOuOJ38
Manually press with the pencil on the membrane. Do you get a keycode? Try lifting the membranes (both) slightly, separating them slightly, and putting them back well aligned. Either of these methods should remove the static or align them again.
If the key works, carefully put everything back together again.
Otherwise, sorry - try again, but you may be out of luck or need to find a specialist who can check the membranes.