XPS cant switch to sleep mode or hibernation

Hello, 

I've seen a similar post here yet I couldnt solve the problem....

My Dell Studio XPS 13 has a strange problem:

When I shut the screen (closes it down) the computer doesn't switch to sleep mode or hibernation, and instead only the screen turns black. the "normal" lights in the keyboard remains on, and the side lights indicate  the computer still runs full power. you can tell that by hearing the regular fan sound.

When trying to to put it to sleep by start = > sleep/hibernate - the same occurs - computer still working but screen turns black.

Basically from that moment on it's stuck until I force shutting down (by shut down button).

Should I update BIOS? would it help?

I'm using windows 7.

Thank you...