Does the switch pull and click like it is freely moving? Could be a loose wire or the switch. If you can't pull the chain, it could be caught on a feed through if a domed light or on another part of the fan. If not, the switch is probably bad, not uncommon. You need to drop the light fixture part, remove the old fan switch and install a new one. If it is , or was, an on-off light, you only need a single pole. If it controlled different lights with separate pulls, you will need a more complex switch. Take the old one with you.