I'm assuming your problem is with the fuel door latch, and if you're lucky and the nut just came off the fuel door latch part of the cable, you can get hold of it to fish it back through without taking anything apart.
If your car is like mine, theres an access panel all the way in the back on the right hand side (theres an interior light just above it). You can get to the cable and fish it back through with some mild profanity

. Do NOT remove the fuel latch from the cable or bend the cable too much (see below)! If threads come through, you're in luck and can just get a new plastic nut from a junkyard 240.
If theres no threads, you'll have to take a lot of interior panels off and remove the rear seats to run a new cable. The cable crosses the car under the rear seat, so if you start there, you can figure out what needs to be removed.
I'm pretty sure the reason I still have problems with mine is because when I went to the junkyard, I removed the fuel door latch from the cable and tried replacing just that. Then I found my cable was broken where it attaches to the latch, so I went back to the junkyard and got the same cable I'd taken the latch from. Since then I've found pics of the cable and none have shown the latch separate from the cable, so I guess they're a unit you're not supposed to disassemble. Mine works to open the fuel door, but if I try popping the hatch with it, the cable pops out of the fuel door latch. One of these days I'll get a new one and install it right, but for now, I'll just use the key when I need to get in the hatch.