This could be for one of two reasons.

1. For your safety, if the jar is removed from the base during blending, the motor will shut off. This also applies even if the jar is slightly lifted unintentionally. 

2. The built in fuse protection has kicked in.

To turn the blender back on, first turn the control knob to "Off" and unplug the unit from the wall. Then remove the jar from the base and free the blade of any obstructions.  Next, press the external resettable fuse button on the bottom of the base. Wait 30-45 minutes for the unit to cool.