Go to LW's prefs & select the iTunes section ... and untick option to use iTunes, apply. Then close prefs. Then go back & change it back. This often does the trick. BTW if LW has been used for a long time such as a few days, this issue can also happen.

Something else ... if your LW is heavily loaded, it can maybe do same thing. So ... Tip: Purge the Downloads Queue! (click on blue link)