Holding the menu button for 15 seconds WILL work - but there is a caveat!
In order for this to work, you must have a cable connected to the selected source with video input. For example, on my monitor, the selected source was VGA and I could not switch it to DVI. So I connected my U2711 to the VGA port on my laptop, switched the laptop on, and then held the menu button on the monitor for 15 seconds. Bam. Unlocked right away. I had tried unlocking all night without any luck. Only when I plugged in a VGA cable to the monitor with input was I able to unlock it.
It took me so long to figure this out that I took the time to create an account with Dell to share my findings. Best of luck unlocking your monitors!
I have a U2711B and the lock button started appearing for some unknown reason. For me, holding 4+5 did not work, but then I just tried holding button 5 (the bottom button) for about 15 seconds and the display unlocked.
I had this problem with a Dell U2711 screen.
None of the above techniques worked. But then I stumbled upon a solution.