I upgraded a PC from 14.10 to 15.04 today with no errors, and rebooted. Now I'm stuck at GRUB. If I select 'Ubuntu' to boot, it reboots the machine. If I go to 'Advanced options for Ubuntu' and then select an older kernel, it reboots the machine. If I go to 'Advanced options for Ubuntu' and select '(recovery mode)', it reboots the machine. If I select 'Memory test (memtest86+)', it...actually runs a memtest (all clear, by the way, breathe easy my friends).
I can get a command line, but I just can't boot the OS. Do I reload the GRUB? Use boot-repair? I've been looking for the last few hours and have found a ton of posts kinda related, but not quite the same--and they all suggest something different.
EDIT: I have tried boot-repair, to no success.