[Alt][Tab]: how to I restore it?
 

I accidentally disabled the [Alt][Tab] function ( I mean the combination of keystrokes with which to switch between applications on desktop)

How do I restore it?

kinda depends what window manager your'e using... please try and provide a suitable amount of information when posting questions.

i use GNOME on desktop

still didn't find out how to enable the [Alt][Tab] switching feature

It's OK now. [Alt][Tab] is functioning as a switcher.
But I didn't do anything to it.
I wonder if this is a bug in the Window Manager.

Also, I noticed that Linux GUI (I'm using RH8 GNOME) is slower than Windows.