permissions - System in low-graphics mode

Possible Duplicate:
How to fix “The system is running in low-graphics mode” error?

I am totally new to Linux, and Ubuntu.

I bought ASUS X53E Notebook, erased Windows and installed Ubuntu. First it worked ok.

Then when I started working with it, I opened the terminal, entered sudo chmod 666 /usr and then all the icons from the main panel disappeared + the whole system stopped responding.

I decided to restart the system. When restarted, a message appears:

the system is running in low graphics mode

and below it:

Your screen, graphics card and input device setting could not be detected correctly.
You will need to configure it yourself.

But the "OK" button is disabled and if I press any buttons nothing happens.

If I enter Ctrl-Alt-F2 it opens the bash terminal. But there commands sudo or apt-get are not found and it says that permission denied if i try to enter any folder like cd /usr

If I enter the su command it asks for the password I don't know.

When encountering this problem first, I reinstalled the whole Ubuntu. but today it happened again just the same.

What shall I do?

You have just corrupted the permission settings of important system files. You won't be able to use the system normally.

By using 666 as permission settings, you successfully removed the execution bit from the /usr/ directory, so no files in it can be accessed. And as far as I know, the unity program resides in /usr/bin directory which is of course failing to execute.

The only option is reinstalling Ubuntu again. And please don't chmod system files without knowing exactly what you were doing.

Check these links:

The last link will give you information about the system files and folders