Normally this issue happens often when you do a Bios update, and you have 2 screens plugged into your computer.
@ the windows welcome/login screen , you will notice 1 of your screens turned off.
Visit your motherboards website.
Example: Mine is a Biostar B75MU3+ Ver. 6.x
Make sure you get the latest onboard graphics driver.
What you want to do, is install the latest graphics driver.
For example I have the Intel HD Graphics driver For a I5 3570k cpu 4000 HD
Once you do this.
Right click your desktop, click screen resolution.
you should see 1 or 2 screens, click the Detect button and Identify button.
Then you will see 2 screens. @ this point of time... 1 probably won't be getting signal.
But it will still show up in windows.
Make sure that you set the highest resolutions, and for the Multiple Displays: dropdown box
make the tab say Extend these displays.
also checkmark the box if you want the certain screen to be your main display.
click ok.
Next in your Intel HD graphics System Tray / driver program.
click graphic properties, and go into advance mode.
On the left side of this new window...
In the Display section, you should 5 choices, 1 is Multiple Displays.
Click multiple displays, and on the right side, you should
Operating mode ( make sure this is changed to Extended Desktop )
Make sure you set the primary display to the correct screen, and set the secondary display.
Click ok, and your 2'ed screen should now turn on :D