The motor mount/water pump bolt froze in the engine block.
When dealing with a broken off bolt there are a few different ways to fix this. One of which involves drilling out the broken off bolt and extracting it using a reverse drill bit. Another way is to drill out the bolt and re-tap the hole to a larger size. You can also take a welder and slowly weld a tower so to speak in the center of the broken bolt and then center a standard nut over the tower, then weld the center of the nut full while it is around the weld tower. Doing this recreates a head on the bolt and at the same time heats up and breaks loose the bolt in the block. Once welded properly it can be unbolted with whatever size socket that the nut is that was welded to the broken bolt. I hope this info helps in some way.