What type of transmission is it?
For most transmissions, if you look at it, you'll see a basic Forward-Neutral-Reverse lever. The cable that connects to it has the sheath locked in a clamp a few inches away and the cable, itself, attached to the level. As you move the controls on top, it just pushes and/or pulls the cable which in turn pushes and/or pulls the lever. The rigidity of the sheath and of the wire itself is what makes it more effective than pushing a noodle. The sheath has a lot of linear stability and the cable has enough for the few inches it sticks out at either side.
So, take a quick look. If the sheath is split, or bits of wire are poking out, or there is an obvious problem with the cable, you might have found the problem.
If not, find the end of the cable attached to the shift lever and disconnect the end of it from the lever. There will likely be a bolt or clevis pin going through an eye on the end of the cable and through a hole drilled in the lever. If it is a bolt, it'll have a nut on it. If it is a clevis pin, it'll have a cotter pin. The eye is normally threaded onto the end of the cable and spinning it adjusts the length -- so try not to spin it while playing with other things. Feel free to use a piece of masking tape to prevent that from happening, or mark the existing adjustment with a sharpie, or just be careful. Good photos before disassembly, from all angles and at a variety of zooms are often helpful.
At any rate, disconnect that end and see if the lever moves. If it does, the problem is unlikely to be the transmission.
Then hold the sheath of the cable at the transmission end, have someone move the gear shifter up top, or try to, and watch the cable at the transmission end. . Maybe it was just bound on something and disassembling it fixed it. But, that is probably wishful thinking.
Then go up top, unscrew the gear shifter and lift it up. Or, get access from underneath it. The end of the cable there will likely be a similar arrangement -- a clamp holding the sheath and a the end of the cable attached to an arm coming off of the shifter. Make sure nothing is mechanically blocking things there. If something is stuck, or obviously broken, you've likely found your problem.
If not, disconnect the cable from the arm under the control. Disconnect both the clamp and the wire. Drop the whole cable out the bottom. Hold the sheath and try to move the wire. If the wire doesn't move, there is a problem with the cable. If it does move, have someone else move it and watch underneath. If the bottom side doesn't move, the cable is the problem. If you determine that the cable is the problem, trace as much of it as you can visually and look to see if you can find a problem, e.g. a really sharp bend, or a split in the sheath, or wires sticking out fo the sheath, or a screw put through it, or a connection to a help selector that might be a problem, etc. If you can find the problem with the cable, you may be able to address it.
If not, you'll probably need to replace the old cable. Look to see if you can figure out how long it is. It is ually stamped or sharpie written at one end or the other. Order a replacement. I recommend spending the extra few dollars for the xtreme or whatever top-shelf variety. Then use the old one to pull the new one back through or wrestle it back through, set the distances at each end to match the old one, and reinstall.
If the cable isn't the problem, try to move the control If the control isn't working, see what is jamming it to see if you can fix it, or replace it. If it starts working, reinspect the able and check to see if the clamp may have been damaging the sheath.
If you can't find a problem, put things back together and see if it works. Sometimes whatever was sticking, e.g. something that rolled out of a drawer, has fallen out and things are good.
If things ever start working by magic, try your best to find the original problem. You don't want them to stop working by magic, e.g. when some drawer is closed and pinches a cable.
For "stiff" cable, some people have had luck lubricating them. Do a seach for threads about that. I've always jusr replaced mine rather than messing with that, so you want advice from someone else who has done that on that one, not me. I could only parrot back what I've heard.
Sometimes controls can be made to work better with cleaning and lubrication. After cleaning, you want to use a grease that won't hold dust. But, again, this is usually a remedy for stiff or other-than-smooth controls, not a hard-stop situation.
Good luck!
-Greg
