I had a similar problem with my HTC One. One day the selfie camera started showing only a black screen. The Main camera showed either a black screen or a static image of the view from when I opened the app. The Video camera seemed to work fine.
The first time it happened I backed up the phone (HTC backup on Windows) then factory-reset and restored it. This worked, but created a laborious task in getting all the preferences and screen icons back in place.
The second time, I researched it a bit and found a suggestion : turn off the phone's Fast Boot option then power cycle the phone. This fixed the problem, and it stayed fixed (so far !) even after I turned Fast Boot back on again.
The Fast Boot setting is at the very bottom of the Settings/Power menu list.
Hope you find this useful.