Is this on a PC keyboard or a laptop? First, reboot your computer... (ironically this fixes most problems). Check to see if the problem persists on various applications, Word, Notepad, Excel, IE... If you press hard on the key, does it work then? Try, if a PC keyboard, unplugging the keyboard from the computer and plugging it back it.
Lastly, you can try removing the key itself with a flat-head screwdriver. You may have a dirt or something obstructing the key from making a connection to the keyboard. So removing the key, cleaning it with warm water and soap and then cleaning the connector with a damp (not dripping wet) towel.
There are probably YouTube videos that can demonstrate this.
Thanks but it is on a Laptop. Do you have any other suggestions that I could try? The Zero keys makes connection and brings up a drop-down window
for "undo, cut, paste, etc" so I believe it is making connection but recognising some other command other than zero?
Yes that's perfect. I wanted to find an image of the layout. I see now that the 0 key does in fact have multiple functions.
Do you see a key "Fn Lock"? It should be next to the right of the F12 key. Press that key, then press the Num Lock key (possibly twice) and be sure you can type other numbers. Then try the 0 key and see if that helps.