How can I fix my M Reg Astra central locking?

How can I fix my M Reg Astra central locking? This is a key operated system. the key will open the drivers door and boot, but not the other doors, or fule filler flap. The other doors will not even open by lifting the internal buttons - they appear to be held down in the locked position. I have had to remove the fuel filler actuator, so that we can fill up.