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
- Reconnect the monitor to the PC video card
- Reconnect the monitor power cord to the rear of the monitor
- Switch the monitor on
- You must continue to hold the button #5 in until the unlock padlock appears
There's one important addition to the above steps. In my case, having connected the monitor to my desired input (DVI1), this did not work. The monitor was set to take HDMI input, so continued to display that there was no HDMI connected. It turns out that by repeating the above process, but connecting to an active HDMI input (aligning with the input the monitor was looking for), the process worked.
Dell, this is terrible design.