The bootmgr is on the system partition. If it says it is missing, then the correct location is not being identified during the boot or that location is no longer available.
Things that might cause such a problem:
Corruption
virus
Removing a hard drive with the boot file (may not be same hard drive as OS).
Putting another hard drive with an active partition as primary in the bios.
and probably some others.
Normally booting to the Install DVD and selecting Repair in the second window, then doing a Startup Repair 3 or 4 times fixes the problem. The repair sequence does not restart itself for the multiple runs, you have to do that. But the repair will fix certain things on each run, and depending one what was wrong, the system will...