This occasionally happens to me on an unpredictable basis. The cure I've established is to (a) close Hauptwerk, (b) unplug the offending launchpad, (c) reconnect it, (d) then relaunch Hauptwerk. No need to shut the computer down. I have my 6 Launchpads labelled with their Windows device numbers so I can go straight to whichever one happens to offend. There's no rhyme or reason which I've been able to establish as to why this happens. My Hauptwerk computer is a PC running Windows 7 64-bit. I unplug the USB cable from the launchpad itself. I am running the 6 launchpads through powered USB hubs - 4 into one hub for the left side of the console, and 2 into a second hub for the right side.
Andrew