Is it over HDMI, are all of your drivers (more generally, video) working/installed, did it work in windows 7...
Also, my extra monitor (over hdmi) wasn't detected by default so what I did was run my mouse over the start button (in default view) and move on up to devices, hit "projector", then press "extend screen to external monitor" and, suddenly, it worked fine.
Also, you could try right clicking on the old desktop and going up to screen resolution and see if it even knows your monitor exists.
Last resort is to pull the plug out of the back of the monitor (both HDMI and power) and make sure they aren't damaged by trying them on other appliances (if the monitor turns on, then the power isn't the problem). Finally, make sure that windows 8
installed the ACTUAL Video card drivers, not basic ones by going to your video card manufacturer's site, searching for your graphics or video card model, and downloading the windows 7 equivalent drivers. If ALL of this doesn't work, then try to reinstall
windows 8 with the external monitor connected during initial setup as windows may be able to detect it much better if it remembers it from the beginning.
Maybe your monitor it too high a resolution for windows 8 to handle (mine's 1080p and it DOES get detected so...) or maybe the monitor imput is set onto some other standard than you have plugged in at the time.
Proposed as answer byDyskmasterjWednesday, September 14, 2011 5:14 PM