Check the Disabled Items list.
Whenever an office application crashes and an add-in was deemed to be at fault, it is silently added to the Disabled Items list. Click the Office button, Project Options, Add-Ins, select Disabled Items from the Manage list and click Go. If you see your add-in, select it and click Enable.
Here is an MSDN page that discusses the issue within the context of debugging your application, but the steps to enable are the same for the end-user.
Microsoft Office applications can disable add-ins that behave unexpectedly. If an application does not load your add-in when you try to debug it, the application might have hard disabled or soft disabled your add-in.