Things to try:
- Check that both wires from the switch to the motor are still connected at both ends, and have not snapped.
- Open up the switch (with care; they're not designed to be opened up). Clean the contacts, and smear a bit of Vaseline on them (to repel moisture in the future).
I had exactly the same problem as you, and it was caused by the contacts in the switch being manky. A voltmeter or ohmmeter is your friend, for figuring out where the problem lies.