If one day you are facing the problem that I am having now , the following information might be very useful.
I am using Fedora 12 and I suspect the root partition some how corrupted or facing bad sector issue power failure.
Here is my solution:
1. Boot from Fedora 12 installation CD or DVD , select rescue installed system option.
2. Skip the mount volume step. You can't check file system when a volume is mounted.
3. When you are at the shell prompt, issue the following command. It will activate the LVM volume.
lvm vgchange --ignorelockingfailure -P -a y
4. If you don't know the volume name , you can use the following command to list all the LVM volume.
5. then you can issue the following command to check for bad sector. I assume you are using ext4 file system for root partition.
fsck.ext4 -c /dev/VolGroup/logVol00
Hope this will help.