To fix a bad superblock, execute e2fsck with alternate superblock commands like e2fsck -b 8193 or e2fsck -b 32768. However, if you experience frequent failures, it could signify hardware issues, suggesting that you should back up data immediately.
First, get a root console (e.g. via grub) and pull up a list of partitions: And from there you take the one you need (e.g.: /dev/sda3). Choose carefuly! And then, you got it: The superblock could not be read or does not describe a correct ext4 filesystem.
Unfortunately, I was unable to recover the file system and had to resort to lower-level data recovery techniques (nicely summarised in Ubuntu's Data Recovery wiki entry), of which Sleuth Kit proved most useful.
Repair Corrupted EXT4 & EXT3 File System You can repair a non-root corrupted ext3 or ext4 file system on a running Linux system. fsck works as a wrapper for the fsck.ext3 and fsck.ext4 commands.