I've had the same problem. Apparently it happens with Dell computers very often. Simple fix however. Make sure to turn your computer off, unplug everything, power, usb connections, dock station, etc. pull out your battery for at lest a minute and while you have your battery out, push and hold the power button for about 10 seconds. This is called "draining the flea power". After a minute put your laptop back together and turn it on and volia! I've tried it and worked like a magic. Let me know how it goes.

I have the same problem on my xps but this trick only worked once for me.  see 'convenience keys not working' in this forum