Try holding in the power button for 10 seconds to reboot. Plug out and wait a minute. Plug in again. You sure there are no shorts on the speaker wires?
If the proposed reboot does not work, un-screw and take off the top of the unit and check the fuse. The fuse is mounted on a small board right at the power cord connector. It should be a size 5x20mm small fuse with a 15A rating. You can check with a digital multi-meter or just order some replacement fuses from Digikey (recommended use LittleFuse brand).
Power switches often only power a relay that does the heavy lifting. Sounds like that circuit may be faulty. Not generally a user serviceable situation.
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