The Dell U2711 on screen display (OSD) can be locked - I think by holding the button immediately above the power button (referred to as button 5 in some places). In my case I locked it totally by accident, and without realising what I'd done.
Unlocking the OSD proved quite the production. I found a [forum post] that contained most of the answer. I'll repeat the useful bit below:
- Switch the monitor off
- Disconnect all monitor cabling from the PC (VGA, DVI, HDMI, DP, USB, etc.)
- Disconnect the monitor power cord from the rear of the monitor
- Press and hold the monitor power button in for 5 seconds