This is one of many solutions to your question,
Did you upgraded to Ubuntu 11.04 from 10.10? If so kernel images of 10.10 should still be on your system. first run,
sudo apt-get install startupmanager
It will install the 'startupmanager', a useful tool to tweak system settings easily. After you've installed it, run
You will see the main program window and under it's 'boot options' tab, you will see 'Default Operating System' dropdown. Choose your desired kernel image (provided your system have the Image) from that list and close the program. Reboot your Computer.
To check wether you booted the kernel you wanted, run this command in your terminal
Hope this helps.