Had the problem before that was solved, but it has suddenly returned. I can't believe it is related, but my wireless keyboard transmitter died. I used an old USB keyboard, and when I started up no OS was found.
Hitting F8 brings up a DOS window I had never seen before. It asks on which hard drive to look for the OS. When I select C, it finds the OS the machine then boots fine. Shutdown and restart and it again can't find the OS.
Disk manager shows both hard drives as active. "Mark partition Active" is grayed out in both.
I have run Startup repair many times. It always results in:
Root Cause found
The partition does not have a valid system partition
Repair action: Partition table repair
Result: Completed successfully.
Then running it again it finds the same error.
the old thread on the problem here: No OS on restart as comp tries to boot from USB drive.