I also experienced this problem, not a crucial VM at all so the loss didn't actually matter to me except for the inconvenience of setting everything up again.
The upgrade process from 10.10 to 11.04 seemed to hang on something 'video' related in the upgrade terminal, although I haven't got a note of exactly what it was. The only fix seemed to be to kill the VM and start from scratch (on restart it had obviously 'upgraded' the boot drive sufficiently far that it would no longer start).
I think it would be reasonable of us to ask for an update on the Parallels update including PT for 11.04 at this stage as well. While I appreciate it has only been on 'full release' for a couple of weeks now the GM versions have been available for much longer as part of the predictable release schedule.
The 2D Unity interface appears to work very well though, or Gnome 3 (although I haven't yet tried it).