This happened to me last year.  I ended up using a small sissors and a pair of tweezers to snip and then unwind a bit of thread at a time as I just couldn't find the end once I'd unwound all that came off easily. 

I was very careful not to cut tiny bits so it woudn't drop down inside (if that's possible) in the gap in the casing and unwound every bit that I cut until eventually it was all clear.

I haven't had any issues with it since I did this and now make a point of watching carefully where the thread is winding.  Still don't really know why it happened, but I was thankful that I always wind on slow speed so there wasn't as much thread as there could have been round the spindle!