There are several ways to test whether or not your HDs are working... if you're able to go into BIOS and check, see if your drives are detected in there... if not, then chances are there's a connection problem. Remove the drives, and plug them into another computer, using completely different cables.
If the drive is STILL not detected, there's something wrong with the drive itself. If it is detected, but cannot boot, then there's something wrong with the software on the drive, and it's best to format the whole thing and try again.
If, however, everything boots and everything works, there's a connection problem on your own computer. First, check the cable.
The cable could have been screwed up somehow when you unplugged it. Better to replace it with another one. Check the power as well. Remember, if either of these cables have problems, your drive will not be detected.
Other things to make sure are the jumpers on the hard drive - make sure they're correct.