If there isn't enough shaft left to use a pair of needle nose pliers to remove it, you can either replace the motor or if you are into DIY projects, you can remove the bottom cover on the motor, and drill through the bottom and push the broken shaft out.
I used this method on my AC110 filter with excellent results. You will just need to use a drill bit that is smaller in diameter than the shaft and then plug the hole with a dab of epoxy. I actually used 100% silicone with good results but it did need to be replaced after a few months with epoxy.
The reason the shaft probably broke is because it became worn over time, causing it to snap off.