The usual problem is when someone has the brake backplate off and doesnt put it back on properly, if you remove the backplate you will see a revolving tab that is supposed to engage into a slot, poeple dont realise that the tab and slot have to be carefully aligned when replacing the backplate, this then bends the tab and the speedo doesnt work, you'll have to have a look to see if this is the case with yours.
...Paul