I have read various threads about wireless disappearing from the Network Manager, and most of these ended up being fixed by commenting out various lines from /etc/network/interfaces . I have tried this numerous times and it's not working for me still.
I have been running ubuntu on my dell for about 6 months and using wireless the whole time with no problem. Recently, I unchecked the "enable wireless" box in the Network Manager because I didn't want it to auto-connect one day. After that, it completely disappeared, not only from the Network Manager, but also from Network Settings,so I basically see no wireless options anywhere where they should be. And, as stated above I commented out everything in the /etc/network/interfaces.