My computer won't start up?

A few months ago, my pc was running perfectly until my sisters boyfriend suggested that I clean it because it was getting dusty. We took the whole thing apart, cleaned it, and put everything back together. Now, when I turn it on, it goes to the screen that has the boot menu and BIOS options. After that screen, it goes to this blank screen with a blinking underscore at the top left corner and stays there. It doesn't get past that screen. If I try to go to boot menu or BIOS settings, it freezes the screen and doesn't even get to the screen with the underscore. Help?