If your system abruptly loses power, or if a RAID card is beginning to fail, you might see an ominous message like this within your logs:
EXT3-fs error (device hda3) in start_transaction: Journal has aborted
Basically, the system is telling you that it’s detected a filesystem/journal mismatch, and it can’t utilize the journal any longer. When this situation pops up, the filesystem gets mounted read-only almost immediately. To fix the situation, you can remount the partition as ext2 (if it isn’t your active root partition), or you can commence the repair operations.