Shut the computer OFF and open the case. Check to be sure that the power cables AND the data cables that come from the power supply and motherboard (or IDE controller expansion card) and go to the HD / Optical / Floppy drives are all fully seated into their respective connection points. Some data cables (SATA types in particular) for HD and optical drives are easily dislodged from the jacks on the drive and controller. Make sure you don't disrupt any of them while checking. This may be a good time to vac / blow any accumulated dust out of the case. Make sure the power supply ventilation slots and fan are not clogged with dust and dirt. The same holds true for any other fan and heat sink on CPU, video processor, MoBo chipsets, etc. Dust will cause interfere with cooling and cause these components to run hot. Heat will cause erratic operation.
If your drives are controlled by an expansion card (ie: not from the motherboard itself), you may wish to remove and re-seat the card in the motherboard slot. This helps to re-establish good continuity between the slot and the card contacts.
When satisfied that the cables and expansion cards are connected properly, replace the cover and power up. Enter the BIOS setup page and make sure that all of the drives are listed. Make sure that the your boot drive appears in the list of drives your computer will seek at boot time. As long as the drives in the boot order do not have bootable media or sectors, the computer should eventually check your boot drive. Of course, if you set the boot drive as the first bootable device,the boot time will be reduced.
If you can't boot your normal boot drive, you should obtain a bootable image and place it on media that matches a bootable drive on your computer (USB, Floppy, CD or DVD). You can download boot images for Windows / DOS / etc. here
free of charge.
Once you have booted from one of these images, you can proceed to test and optimally copy data from a drive the will no longer boot. If you are unable to access the original boot drive, there may be a problem with the cable, power, controller or even the disk itself. Your ability to recover the data will depend on how much time & money you want to spend on doing so.
I hope this helps & good luck!