From Repair Mobile

How to fix a broken key on a cell phone

It depends on which kind of key, you didn't clarify so I'll describe how to replace two different kinds of broken keys on most cell phones.

If you'd like to offer more details then your answer results will certainly improve.

Also, this answer assumes your phone can't be serviced under a warranty or equivalent provision which would be simpler since replacing the entire component instead of repairing it is preferred. More details [1] - thanks to amazon.com