[ubuntu] Upgrade to 16.04 issues

I was ruing 14.04 on both my laptop and desktop. A message popped up to upgrade to 16.04. I did this with this laptop on Saturday with no problems and it is working fine. I did the same with my desktop today. I now have a big problem. The upgrade seemed to take a long time (almost 4 hours). It reached the end, cleaned up files and asked if I wanted to restart. I clicked yes. I'm now stuck with the purple ubuntu screen with the 5 dots undeeath. It cycles through the dots twice and then stops with all the dots highlighted. I'm not sure what to do as there is a significant amount or irreplaceable data.

Any suggestions about how to restore access as the whole thing is just frozen

If there isn't a solution to this. How do I go back to 14.04 or re-install 16.04 if the computer is frozen? The boot menu has 10.04 (presumably the last time it was updated from disc). I can't find an option to upgrade from that