Troubles with the rhythm/treble switch?

There are only two possibilities...
The rhythm pick-up has gone open circuit; would need to be replaced, or more likely the changeover switch has dirty contacts; can be squirted with switch cleaner and moved rapidly from side to side to clean the contacts. That's assuming you can get to the contacts at the back of the switch.
You could also do a bit of maintenance at the same time and clean the volume and tone controls as well.
Hope that helps.

Guitarman
August 2008