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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?