I assume you tried to press the test flash button.
If that does not work AND it's not working when mounted, it's probably toast and would need repair/replacement. There are several high voltage capacitors that could go out, and the flash bulb itself can go bad.
If the test button works, but it still won't fire on a camera, it's a problem with the connection on the hotshoe somehow... that's different, but still usually requires repair (unless you can just clean up the connector)