That will depend on the why the hard drive is no longer being seen. It could simply be a need to replace a bad cable unless the read/write heads on the drive were going or gone. If you were inside the case at all recently you may have bumped the drive cable with it simply having been pulled out a little to see that be another prolem.
Simply changing the boot order won't effect data on a drive as long as that was intact to begin with. If the drive is not being seen on the bios post screen the cable or drive if not simply a jumper setting for an ide drive oculd well be going bad on you.
First examince the drive's data cable as well as the power plug to see if those are plugged in fully. The next step if found plugged correctly would be swapping the data cable and trying a different power plug to rule those out if not found bad.