Tried to update from Ubuntu 10.04 LTS (used for 3 years) to 12.04 LTS on my Dell desktop using update manager now get grub prompt. What do I do now?

I am a relative Linux newbie who has tried to update from Ubuntu 10.04 LTS (used for 3 years) to 12.04 LTS on my Dell desktop using update manager, thought all was working well, even though my HD partition desktop is small (just 20gb) and commentary stated short disk space. I am now faced with the sole grub prompt, GNU Grub version 1.98-1ubuntu13 and message ¨Minimal BASH-like line editing..¨ Could anyone please advise how to progress step by step? It´s my main computer I am worried I won´t be able to access my data (partially backed up). Can I try to go back to 10.04 or recovery mode, or boot from USB? Any help appreciated.