When I turn on my computer.. there is sound but its normal sound and my screen stay blank... and I can't see nothing. So anyone knows what's the problem?

Maybe you need a new monitor. If you computer is still running but the screen isn't coming on that is what I would guess.

Have you had to move any of the connections? If so I would check and be sure that the plug for the monitor is nice and tight. It will look something like this:
http://www.firebird-systems.com/graphics/tuners/cmt345_files/dual_vga_cable.jpg

Also how many slots do u have for a monitor on the back of the PC?

Thanks for the answers, it was something with the USB ports, not sure what was the real problem though xD