Background. I have a MacBook Air (13-inch, Mid 2012) Version 10.11.4. I own a 27" BenQ GW2765 monitor. I have had few problems using this monitor for ~2.5 months - using it at maximum resolution. There have been blips where the monitor has not been detected, but I just shut both the laptop off and monitor off for a few minutes, and the problem is resolved. Last night, I left my set up all connected, my laptop in sleep mode. I woke up this morning to find my screen in 1080p. I thought this was a bit odd, so I tried to change it back to 2560x1440. And the screen just changes (see picture). Every other resolution seems to work. From 480p to 2048 x 1152. What I can't get my head around is why it won't work on 2560 x 1440 anymore. I have no updated anything, didn't download anything. I've tried changing the miniDP-DP cable, SMC/PRAM reset to no avail. Does anyone have any clue why this is happening? I've googled it, but cannot find a reason (nevermind a solution) to why this is occuring.