Your monitor has an 'OSD' -- On-Screen Display, which can control the monitor's hardware.
That OSD is displaying one of the options for the display.
Look for 4 buttons along one of the sides of the monitor:
* activate OSD
* "left"/"up"
* "right"/"down"
* "accept"/"exit"
Also, look on the bottom and top edges.
Rebooting will not deactivate the OSD -- power-off the monitor, and then turn it back on.
Or, look for one of those 4 buttons to be "stuck".