Do you know if it was a "mechanical" fault - the needle simple got stuck - if so you can get special electrical none conductive maintenance spray from electrical suppliers - not WD40 type stuff you get from B&Q! If an electrical fault - disconnect the battery for 15 minutes to let any residual charge in the system disapate and see if that works. Alternatively, take the instruments out of the car, disassemble as much as you dare and just give it a good blow to try and shift any fluf and dust!
Spartan535's post in Lupo Revs Stuck was marked as the answer