Possible causes of this:
1) Partition/drive boot flag is missing/corrupted, & or the active status is incorrect.
2) Newer BIOS can boot a PC from power on so fast, the drive has barely spun up (much less initialize) & the drive may be skipped.
3) If you have other connected drives with bootable OS/Flag/active partitions, BIOS may get confused & attempt to boot from wrong device, causing problems.
4) Bug or flaw in older BIOS.
If 1 is the problem, you can install via your installation DVD instead of doing a partition "restore". Backup anything important before beginning.
If 2 is the problem, you can set the BIOS to have an extended POST & ensure your drive priority is correct.
If 3 is the problem, you can disconnect extra drives to prevent problems. Use external drive bays after the OS has loaded instead of loading with the OS.
if 4 is the problem, you can update to the latest BIOS for your model, being certain you are on battery backup protection before beginning.
There are other possibilities.
What I really wonder, is if the way you do things has interfered with the normal boot flag/bootable drive or partition?