We had this same failure on a C-36. It is a pretty cheesy way to hold a gooseneck on. On ours the pin literally sheered leaving the pin hole full of pin and the other ends of it inside the boom. I over drilled the pin hole to use a larger pin and also made a new washer with a tighter tolerance.
Not really the most robust gooseneck design.....
Here you can see the sheered cotter pin still in the hole....

To get back to port I re-inserted the gooseneck into the boom then made a boom "inhaul" using the green line pictured and a truckers hitch. I took two reefs, rigged a preventer and we made it without issue despite it gusting to 25 knots..

Once in port getting the end cap out of the boom proved pretty tough as the factory did not use an anti-corrosives like Lanolin or Tef-Gel on the screws. Once the end cap was out the fix was easy. The hole was pretty close to the end of the pin but had enough meat to go to the next size up pin. I found the washer WAAAAAY to sloppy and this is likely why it sheered the pin as there was too much slop.

But one of the small details that make these boats great coastal cruisers but not the best suited for ocean crossings.... I mean really, how much did they actually save on this cost cutting design flaw......
