How do you open a locked filing cabinet if key doesn't work?

Evaluate what is the cause, could it be that your key is overused and you require a new one? Is it possible that you have the incorrect key for the cabinet check code on key is the same as on the lock face. Also could there be anything in the lock that would stop the key from opening the device.

A simple fix would be to try using some WD40, or you could ask the guys at www.fastkeys.co.uk they might be able to help?