Update - as it turns out, this may have been due to a hardware fault (which memtest did not detect at the time, but later did). No way to tell for sure. Hasn't happened for a while now.
-- [4294668.613000] Kernel panic - not syncing: VSF: Unable to mount root fs on unknown-block (0,0) https://launchpad.net/bugs/29319 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
