The same exact thing happened to one (and only one) of my turbo fins. I immediately ordered some replacement pins and rings (and rudder pins) to carry on the kayak at all times.
In the mean time I took some pliers and reshaped the ring as best as I could. It failed shortly afterwards. I was lifting the drive out of the drive well and noticed one fin was half way down the shaft. The pin and ring were gone. Luckily I had already received my replacements!
Make sure you have an emergency spare parts kit onboard!

My kit has a spare set of Turbo Fins, 5 or 6 clevis pin and ring sets, and 3 rudder pins.
One potential place to order spare parts:
http://www.austinkayak.com/catalog_sear ... evis%20pin