SOLVED my own issue
Originally Posted by markhouston72
Did you have any luck solving this one?. I am having the same issue as you are.
Totally stock (un-rooted) HTC One M7
HTC Sense 6.0
After upgrading to 4.4.2 about a month ago the camera stopped working immediately. When you open the Camera app you get a black screen, touching the screen where I know the controls are results in hearing the noises as you would expect. I installed Camera ZOOM FX and this gave me camera functionality back, but when I switch to record video I still get the same black screen after about 1 second of seeing through the view finder. All very frustrating.
OK after rooting and ROMing (Android Revolution) my HTC One M7 and still getting the same issue I decided to take a different tack. During the many re-boots during the rooting and ROMing process I noticed that sometimes the camera worked early after a re-boot but stopped working later. So using a task killer app I was able to track down the app that was causing the camera to show a black screen.
Believe it or not it was Ultimate Rotation Control which I was using in conjunction with tasker for some automations. The minute the app started up the camera would immediately fail. So uninstalled and, happy days, the camera is back to working order.
Hope this helps someone else in the future.