Computer can boot mem test from usb but will not boot any os?

I plugged in a second hard drive to install a linux onto it for a computer that did not support usb booting. When I removed the second hard drive to original computer it will no longer let me load an operating system. It just goes to cursor on an otherwise black screen. I downloaded ubuntu onto usb and it boots to the menu where I can select try ubuntu live from cd, and it also gives me an option to do a mem test. The mem test works as it is just simple text on the black screen like dos. But when I try to boot any operating system, including my windows vista installed on original hard drive and the usb ubuntu live it gets stuck on the cursor flashing at the top left of screen. Why does it do that and how can I fix it. I have a 2005 compaq presario