On startup the system hung because the root UUID was not found. http://ubuntuforums.org/showthread.php?p=9020811#post9020811
So when I ran
sudo blkid — executed from a live cd — it didn't show the root partition (all other partitions UUIDs were listed).
But tune2fs showed me its UUID
sudo tune2fs -l /dev/sda2
So I tried to set a new UUID:
uuidgen # with uuidgen's result: sudo tune2fs /dev/sda2 -U a72d5565-b5fd-4e15-bfa8-8067e6bdfbc8
This didn't help.
... was to set
linux [...] root=/dev/sdaX [...] in the grub2 settings.