N73 - Broken button or configured badly?
 

I have this phone; only had it for a few weeks and this button which i have most skillfully highlighted in this picture doesn't seem to do anything anymore.

I first had it working when i got the phone and was playing with all the settings but now it doesn't seem to do anything anymore... have i simply messed up the settings or is it broken?

Can it be disabled? If yes, how and can i re-enable it again?