Error 21 on Ubuntu 9.04, how do I boot up?

"Error 21 means that the ""selected disk does not exist"". So somehow Grub expects to read data from a disk which does not exist. Since this happens before you're shown the dual boot menu, I'd say that the location of the disk which contains stage 2 (?) is not properly configured. I cannot specifically say how to fix that.

Have a look at Super Grub. Store it on a USB stick/key or burn it on a CD/DVD and use it to boot your PC. It will allow you to recover your PC so that the Grub boot menu works again (which will in turn allow you to boot into Linux and/or Windows). Good luck!

By the way, I found this solution at the Ubuntu forums (specifically, this thread). It is generally a good idea to check those forums if you have any Ubuntu-related issues and/or questions. (Not saying that you shouldn't ask here, but you'll have an increased chance of success over there.)"