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Press power button and volume up at the same time,you will feel 3 vibrations. The device will turn off. Turn it back on.
@uliwooly I think I remember something about this since I have used Xperia P.
@alexdon Yeah the NXT series seem to have the same issue.
Not sure but it happened quite a lot on the P,never on Z1 though (or no date that I can remember of).