This happens occasionally where a window or panel 'drops' off the edge of the screen making it impossible to select or move (although usually it can still be resized ::) ). To fix, find "studio.xnf" in your 'user' directory and delete it.
For Vista users this is located in the following folder (assuming default installation);
C:\Users\[user-name]\AppData\Roaming\Winamp
For Windows XP users it's located in the following place (assuming default installation);
C:\Program Files\Winamp\
Next time WinAmp is opened the 'theme' will have been reset back to defaults. Note that it will loose the theme you were using before the problem happened so you'll need to re-apply it.
