If you say it went up to 20, stopped and won't return to 0 the mechanism is hung up on something. If it goes beyond 20 and only returns to 20, it's not the spring.
Take it out. The bezel is held on by the edge being crimped over the can in a couple places - use a screwdriver to straighten the crimps. Remove the bezel. The mechanism is held in by two screws on the back of the can. Remove those and the mechanism is new exposed enough to examine.
The input shaft should spin free and the nothing should be rubbing metal to metal. If it is, you're better off just getting a working unit because that means something is knocked out of alignment. If it's rubbing on something you can remove (like a dead bug) remove it.
Be Extremely careful with the pointer needle. They are usually plastic and extremely fragile because the plastic becomes brittle over time. Replacement needles are available but unless you have the patience and deft hand of a watchmaker, they are VERY difficult to replace.
Any questions, PM me.
