You can no longer adjust output voltage? This could be a bad connection at one of the potentiometer terminals.
If not then it is the control circuitry, which could be anything.
Are you saying it can be turned around and round, without reaching the end of travel? This sounds like a broken stop inside the potentiometer.
You'll need to replace it. Or if you wish, you can disassemble it, and see if you can attach (or re-attach) a small part to either the rotating part or the stationary part.