Manual transmissions can be fun to drive, but a problem with shifting is no fun at all. If your vehicle is stuck in reverse, it could be a simple issue with the clutch or a larger mechanical issue.
A transmission can be stuck in reverse or another gear if the clutch cable isn't extending properly. A simple remedy is to pull the clutch pedal upward slowly and firmly. However, the cable may be badly damaged by rust, grime or overuse. Replacing the cable can be necessary in this case.
The transmission requires a separate set of hardware for reverse since the gears have to move in the opposite direction. Shifting between gears is less complicated mechanically, but the extra step for reverse can make those gears slightly more vulnerable to misalignment or damaged teeth. This issue can sometimes be solved by opening the transmission box and tapping the affected part with a hammer, but severe damage can require replacement.
It's possible that a manual transmission could get stuck in reverse due to a physical problem with the stick shift itself. It may need serious repairs, but if you can take the top off you can perhaps force the shifter into neutral and fix the problem, at least temporarily.