If your mixer's motor was built by Hobart; there are thermsiters built in to the stator's windings that can cause this problem. Check the amp draw while it is running. Also if your are over loading the mixer or using the wrong speed,, that will cause an over heating problem. You should be able to get on the ITW Food Equipment Group / Hobart web site(s) and get capacity chart.
call in an electrician as it may be the start/run capacitor for the motor ( indicated by motor humming but not running)
the gearing will make it very difficult to turn the blades by hand as the motor will be spinning at either 1400 rpms or 2400 rpms down to around 50 rpms for the blades
Google perhaps a fuse is blown something was to heavy or something was under tension to the power supply that so the machine did not get the required voltage by too much taxation by other devices on the power line group