how to install grahic ATI card driver by command line for Acer Aspire 5560G

I am using Ubuntu 12.04 on my Acer Aspire 5560G.
I clicked on set up -> additional driver, and found there was an entry of post release upgraded for ATI graphic card.
I clicked to activate the upgraded driver, but it was not successfully installed.
I then clicked to check the initial version, but found that it has changed to "not activated" too (it was initially activated).
Then I clicked to activate the initial driver version, and it was installed successfully.
Then a reminder said I should restart Ubuntu to use the installed driver.
I forgotten to restart, but have use Bleach It to clean up, and have suspended the computer once, and then shut down.
When I fresh start the computer the next day, I see only blank screen with a cursor, nothing else.
How do I reinstall the graphic driver through safe mode command line, or how do I get back my usual Unity desktop?