data recovery - How to recover a 64-bit Ubuntu 12.04 installation?

Using an AMD 64-bit system. I upgraded using package manager (i.e. on-line). The desktop freezes after login.

How can I recover using a cd version? I do get a response when selecting "Recovery Console" before logging in. Perhaps there is a cryptic command that can help me get the system back on track?

Cheers, ....Erik.