If you share exactly which Black Ops game and whether it's modded or official DLC, I can give a more specific fix.

Since you cannot access the main menu, you must force the game to bypass the missing zone using launch parameters.

If your zone files are present but the EXE refuses to find them, you can trick Windows using a symbolic link.

If you only installed the Multiplayer or Zombies component, the game may crash with this error. Ensure the Single Player