it happened to me months ago and finally fixed it today. The on/off switch consists of a piece of conductive rubber that makes contact with the circuit underneath. In my case the circuit or rubber became misaligned or dirty. [br]
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Either way what fixed it was to disassamble the switch side carefully, remove the switch rubber, clean it and the circuit with some q-tips dabbed with alcohol and reassamble all.
Deassembly was the biggest pain.