Since you cannot get to the main menu, I think you'll need to close JMP, open Windows Explorer and navigate to the folder containing your JMP settings. Most likely one of these:
 
JMP:  c:\users\<userid>\AppData\Roaming\SAS\JMP\12\
PRO: c:\users\<userid>\AppData\Roaming\SAS\JMPPro\12\
 
In that folder, delete homeLayout.xml and restart JMP.
 
For people who can get to the main menu and want to put things back how they were originally, use View > Home Window Panes > Reset to Factory Layout. 
 
Hope this helps,
John