Sounds like the left leg of the three leaf spring has slipped off the sear. You'll have to remove the mainspring housing, thumb the hammer back, remove the thumb safety, and remove the grip safety. This allows you access to the three leaf spring. Set the spring so its bottom flange fits in the slot cut in the frame and the left leg of the spring on the sear, and put the mainspring housing half way on to hold it in place. Then reinstall the grip safety and thumb safety. Put the mainspring housing all the way in, and insert the mainspring housing retaining pin. You should be all set.
Obviously, before you start ensure the pistol is unloaded.
