If you pull back the side felt/carpeting in the trunk, you can manually disengage the lock mechanism at the fuel door. That should at least allow you to put gas in your car and use it until you can troubleshoot the lock issue.
Run through
this troubleshooter. It should help you pinpoint the fault.
The central locking system can be a pain in the ass and is fairly complicated in these cars. A week after I bought my '87, the system quit working. In my car, the power locks completely died - there was no automatic function of anything. You had to manually lock or unlock both doors and trunk. I ended up taking the central lock computer out and took it apart. I found that there is a solder joint in the lock computer that is designed to melt and open the power supply circuit if one of the actuators draws too much current. I resoldered that joint and was good to go. Since it sounds like your entire system died in the locked state, it might be the lock computer causing your problems.
The central lock computer is a pain in the ass to get out. It is behind the driver's side kick panel. You have to remove the driver's side kick panel, remove the speaker, and down in the hole where the speaker mounts, there is a black plastic "tray." You have to remove that "tray," and the computer is screwed to a bracket down underneath it. The black "tray" was a big pain in the ass to get out. I ended up breaking mine to get the damn thing out. If you do remove the lock computer to check it out, be ABSOLUTELY SURE that you disconnect the battery BEFORE unplugging the harness from the lock computer. If you do not disconnect the battery first, it WILL fry the lock computer and you will then need to buy a new one ($$$). When you plug the computer back in, also BE SURE the battery is disconnected.
A few months after I fixed the lock computer, my driver's side door lock actuator died (maybe it was the culprit with the original failure). I replaced it with one off ebay. Haven't had any problems with the locking system since then (except for some adjustment issues with the driver's side actuator).
