Does the BIOS find the drives when the PC is booting up ? - you might find that the easiest way to see the boot screen is to shutdown with the restart option - that way you don't miss it while the monitor is warming up. You could also go into the BIOS setup program and have a look to see if they've been detected. This usually involves holding down a key (often DEL sometimes F2) as the PC is booting - there's usually a message which comes up at the beginning to tell you which. If the hardware isn't finding the drives then Windows won't.

If the BIOS is detecting the drive OK, then follow Amergin's advice, if not then it's most probably one of the internal cables which has gone astray. Most PCs have two IDE ports, each of which is capable of handling two drives. I expect that your hard drive(s) are connected to the primary one and that the CD/DVD ones are connected to the secondary one. IDE drives require two main connecting leads, a 40-way ribbon cable for data and control and a 4-pin power cable. The 40-way cable for the CD/DVD drives should have one end connected to the motherboard and there will be two connectors on the other end - one for each drive (they're interchangeable). The side of the cable marked red usually points to the power connector side of the drive. The 4-pin power connectors have curved corners on one side and will only fit in one way. The 40-pin cable is the usual offender, it only needs to be not pressed in properly at one end to stop the drive working. CD/DVD drives usually have a small 4-pin cable for sound etc when playing back non-data disks, but these wouldn't stop the drive being detected if they weren't connected.