First you should try to start Ubuntu in recovery mode or with a previous kernel by selecting from the Grub menu (appears when holding down Shift while booting). We can also run a maintenance shell where we could remove unwanted applications or update Guest Additions to the current version (in case this was not done already).
It may also help booting to disable 3D acceleration in the display properties from Virtual Box Manager.
In case you are still unable to start your virtual machine after an upgrade you may consider to go back to the snapshot before you upgraded or to restore your backup from the virtual machine before having performed the upgrade to 11.04.
In addition to an online upgrade we now have the additional option to upgrade an existing system by using the installation CD-ROM image. After choosing Install Ubuntu you will have the option to upgrade your system by keeping your personal data and system settings:
A similar option will also be available when booting your upgraded Ubuntu 11.04 from the installation CD-ROM image of 11.04. By this you may be able to reinstall or repair your Ubuntu without deleting data and settings in /home.
Before upgrading it is always recommended to backup your virtual machine files, and in addition to backup the content of your /home directory to be able to restore it later.