How to turn Dell inspiron 6400 Wi-fi back on??

Hi, I have a dell inspiron 6400 and somehow the wifi has been turned off and all i get is a message that say: Wireless capability on this computer is turned off. Turn on wireless capability: This can be done by using a switch, which is usually found on front or side of the computer, or a function key combination. I dont have the instructions with me and can't seem to find a switch on the computer and have tried everything else. how do i get the wifi back on??? thanks nick