How to resolve keyboard issues?

If it is wired, then try a different cable. You can also try a reset the SMC of the computer (Apple has instructions online, the procedure is different for different computer models/years) in case the port is frozen. If the keyboard is non-Apple and has a driver/software, try uninstalling and reinstalling it. If none of these work, you'll need a new keyboard.

If wireless, make sure it is charged, make sure bluetooth is working and the keyboard is detected, otherwise re-pair it to the Mac. If it is non-Apple and came with drivers/software, try uninstalling and reinstalling it.

Do none of the keys work?