I also had this issue. I have a few of these monitors and one had power button locked. None of the solutions found on this site worked for me. But I tried other buttons and the LEFT ARROW button is the one that worked to lock/unlock the power button after holding it for 10 or 15 seconds (my bad, I didn't actually count, I just held them all down one at a time).
I purchased 3 of these monitors some time back. I recently mounted 3 of them onto a 3 arm stand. Somehow on one of them, the power button is locked. I've tried every single thing I could find on the internet to unlock this monitor's power button but the documentation on this specific model is poor and I am out of luck. Any help here? Occasionally I get the "Out of Range" notice that requires a quick power cycle to fix. However on the monitor with the locked power button, I have to cycle through to a different input and back which is extremely tedious and annoying.
D2719HGF, power button locked