I think I may have had the same problem:
I've got no solution but a workaround, adding to the kernel parameters.
BUG: soft lockup - CPU#0 stuck for 61s! [plymouthd:401]
To temporarily add a kernel parameter:
- press 'Esc/Escape' for selecting a kernel at early boot time
- select your kernel and press e
- add nomodeset to the line and press enter, then b
is enough for me to get rid of the hanging boot. Hope this helps some other people, too, since I only registered with ubuntu forums to leave this comment.
(VGA compatible controller: ATI Technologies Inc RV280 [Radeon 9200 SE])