If the upgrade has been performed, there is no way back. But your OS is starting. The Gnome error messages indicate that there is a problem with the graphical interface.
If you know the shell (text console) you can activate that or the recovery mode (AFAIK by pressing ESC during booting to open the Grub console, and selecting Extended - Recovery Mode).
If you don't have a recent backup, use the console to create one now.
In the console you can try to analyse the problem. It might be for example a problem with the drivers for your graphic card.
You might also try to repair the installation (on your own risk):
sudo apt-get -f install
sudo apt-get -f update
sudo apt-get -f upgrade
If nothing helps you have to perform a fresh installation from media and restore your data using the backup. If you don't have experience with the shell, that is the quickest solution anyway.