Sounds like a component or two may have turned "thermal"... that meaning they are sensitive to temperature and act up accordingly. It could even be a bad solder connection or two that's being affected by components that normally heat up after awhile. You may want to inspect the circuit board for such poor connections, especially near heat generating components like transistors and IC chips mounted on heat sinks. Large power resistors are also likely culprits. A hair dryer set on med. to high with its air aimed at specific components may speed up in troubleshooting the problematic connection or component(s). The unit must be on in order to tell when it is working or not. Do not get the dryer too close to the components since many hair dryers have a wire mesh grid at their exhaust port which could short components.
If you are new at electronics troubleshooting, I would strongly suggest that you NOT remove the back of the unit and go snooping around, for obvious safety reasons. Take it to a repair shop or purchase a new unit.