Your Ubuntu system is having a boot problem well before it gets to the login screen; this is not actually a login problem, specifically.
Some problems that present in this way can be fixed by reinstalling GRUB2 to the Master Boot Record. You can do that with any of these techniques. If that does not fix the problem, please edit your question to indicate this, and also provide details about what technique you used for reinstalling GRUB2 to the MBR and what happened when you tried (as if it doesn't work, it's possible that your problem is not one that can be fixed this way, but also possible that there was a problem in reinstalling GRUB2 to the MBR).
By the way, the advice in this answer also pertains to Ubuntu 11.04.