What did you change, and what disk did you insert? It's not just asking for any disk, that message means it either can't find the hard-drive, the hard-drive is not in the boot order, or the files on the hard-drive are messed up. To boot from a disk it needs to be a bootable disk.
Go back to the BIOS and make sure the hard-drive is seen and is listed in the boot order. You may want to place it as the first device.