You have a failing hard drive, or the partition map is simply really messed up.
If you didn't do anything that you are aware of, to 'lock' your drive, then maybe the hard drive is coincidentally close to failure.
Boot to your older system - which would be the system restore DVD, or your previous OS X installer, whatever that is.
When booted to your installer DVD, you'll likely still need to re-partition the hard drive. You'll find Disk Utility in the Utilities menu. Do the same process that I listed to re-partition your hard drive, then reinstall OS X...
Serendipity is a lucky guess !