Originally Posted by gokun
I think you only need to hold Vol Up + Power for a few seconds and it should reboot.
Mine did the same thing once and that's how I got it to restart.
True. Power + Vol Up is a reboot, basically a simulated battery pull (for phones that have a removable battery.
Power + Vol Down boots into bootloader.
Power + Vol Up + Vol Down is not any button combo on this device, that I know of. This is perhaps a button combo for another device? Care should be taken to only do known button combos. Although I don't think there is any "harmful" button combo on the M8; on my old One X (EVITA) folks would mistakenly do Power + Vol Up to go into bootloader (correct combo is Power+ Vol down) which erased the ROM and sent the phone into Qualcomm download mode, which would probably give you a heart attack!
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Phone: AT&T HTC One (M8)
Tablet: Samsung Galaxy Tab Pro 8.4 (WiFi only)