If you were already using 10.10 and you recently moved over to 11.04 then most likely you are getting this issue. One of the options to quickly try out is that can you go to the previous versions of Ubuntu and boot up successfully?
If you can then you are just missing one file which is a bug.
Now what you need to do is that bootup with 2.6.38-8-generic-pae(recovery mode). Log into the safe mode .. which is the lower UI resolution mode.
Goto the terminal and execute
$ sudo apt-get install linux-headers-generic-pae
Once this is done restart and then let Ubuntu boot up successfully on the 2.6.38-8-generic-pae version