Originally I thought the worm gear had gone bad - the most common problem when the mixer head won't turn. Got a new worm gear and took extraapart the mixer (followed the instructions in an online video tutorial). The worm gear was fine! The problem turned out to be that the small metal pin that secures the worm gear and drive shaft together was missing (?). Don't know where it went or if it was ever there in the first place, but without it the drive shaft and worm gear do not work together to turn the planetary. This is a 10 year old mixer and it has worked fine until just recently. Crazy. Installed the pin and put everything back together, and it's fine. Kept the extra worm gear for use in the future, if needed.