how to fix it:
0: ensure your gun is unloaded
1: remove the grips
2: remove the two sideplate screws and the yoke screw and put them in a safe place, ensuring you know which goes in which hole (draw a picture of the sideplate on a piece of paper and stick each screw through the correct holes to keep them straight).
3: remove the sideplate by tapping on right side of the grip frame with a non-marring mallet (i.e. a nylon-tipped hammer). This uses the inertia of the sideplate to your advantage, as the frame will move sharply, but the sideplate will want to stay where it is, and so will work itself loose. DO NOT PRY UP THE SIDEPLATE WITH A SCREWDRIVER, as doing this will mar the edges of the frame and sideplate.
4. loosen the mainspring strain screw (front of the grip frame) enough to relieve the tension on the mainspring and remove the mainspring. Remove the hammer block.
5. remove the hammer assembly (you may have to work the trigger a bit to be able to lift the hammer out of the frame).
6. With a properly-fitting punch, punch out the hammer nose rivet. The broken hammer nose should come out now.
7. Install a new hammer nose and tap the rivet back into place. Use a center/cone punch to flare the rivet into the hole.
8. reassemble the gun, and test its functioning.
You could also use these steps to replace the whole hammer assembly, if you want...