If it worked before, then cable connections are most likely not the problem. If it did not work before with the way it is set up. then tell us how you have each drive set on both the ribbon cable and whether (assuming IDE) you have it on the primary or secondary IDE channel. If Sata, we need to also know that information.
One last thing, go look in the bios setup menu and make sure in the boot area that you have the:
Hard Drive Priority - Boot drive set first
Boor Order - CDRom first, Floppy second, boot drive third
Note: If you change anything, be sure to save before you exit.