[morzh]

Well, is it stuck due to something holding it? In any case you will have to open the meter and inspect the movement. Usually the adjustment is done using a small spiral spring with one end attached to the galvanometer's coil and the other held by some thingy with a bump which goes through an oval slit ina lever rotated by your adjustment knob. The result is the spring tension varied and the spring pulling on the coil setting zero position. You have to see if there is an obstruction.