I have upgraded to 12.04 a few days ago and it was working just fine yesterday. I just tried to boot Ubuntu now for a couple of tries and it gets to he 5 purple dots and stops. Not sure what to do. It is on a dual boot so I have windows but that is not exactly ideal.
Make a bootable USB, boot from it and click on install. During the steps, when it asks for the partition, select the partition you already have Ubuntu installed on. :)