I had exactly the same thing happen last fall. Yep, someone has to go up the mast...
When it happened to me, the 'fat' part of the loop spliced into the end of the halyard got jammed on the sheave at the top of the mast; I had to run a line through the shackle and put most of my body weight on it to get it un-jammed. Once un-jammed, it slid freely to deck level.
At the time, I did not have a spare halyard that went to the top of the mast installed; I had to ride the jib halyard 7/8ths of the way up, retrieve the messenger that was in place for the topping lift, and run the topping lift back to the deck before I could get to the top. It's nerve-wracking to go up without a safety line, but now I won't have to do that again!
Finally, I had my mother on board that day... and she described the day as 'an adventure!'. Then she told me that her definition of 'adventure' is 'any situation where telling the story afterwards is a whole lot more fun than going through the experience'...
