If you have not already done so, with the pc powered *OFF*, disconnect and then reconnect the interface(vga) cable.
Loss of a colour can be caused by a simple poor connection.
If this does not work, try the monitor on another pc.
If still not working, it suggests a fault with the monitor (or cable as Gretti suggests), in which case take it to your local electronic (not pc) repair shop where they check for free, then quote for a repair if possible.
Please advise back on progress.
Good Luck - Keep us posted.