There are many things that can happen that can happen with a blank screen on the computer, but first see if number 1, 2, or 3 is your problem.
Number 1: You don't see anything on the screen at all.
Number 2: If you see insert disk and press enter.
Number 3: If it loads but then goes blank.
To fix the problem
If yours number 1 please follow these directions.
Simple steps first make sure monitor is plugged in to the wall and the tower if that does not work then do this, if you have a video card open the case and make sure its properly inserted in the slot also make sure it has a screw mess around with it just make sure it is in right, if it still fails try unplugging the video card and (if possible) plug the monitor straight into your on board video if this is successful your video card is either not compatible or dead I would call your graphics card company and talk about your warranty options.
If yours number 2:
Then its your hard drive first open up your case then unplug the cables into your hard drive and then back in (this usually fixes the problem like 70-80% of the time) if this step does nothing then your computers OS may have failed (there is also a chance that your hard drive may be dead as well but this usually does not happen) so buy an OS. Once your OS is bought put the disk in and it should say press any key to start it do that and then just follow the instructions.
If yours number 3:
Power on your computer and press F8 a bunch of times tell that safe mode stuff comes up go to enable low resolution (once your in just go to control panel and display and keep putting it smaller tell it can't anymore (don't worry it has a timeout if it does not work) if that won't work go to safe mode and roll back your video driver from device manager ( its in system tools (just right click roll back driver).
No,that is not how.You simply take the O/| part on the back of you computer(found just below your power cable) and put it on O.Hold your power button(now located on the front) and switch it back to |.(Still holding it) Now it should be up and running!