What were you mixing and for how long?
Some mixers have an internal Thermal Overload Protection switch which will turn the mixer off if it overheats. Your mixer may have overheated, depending on what you were mixing and the length of time the mixer was in use. Once the mixer has cooled down completely (about 30 minutes) the mixer should be able to be turned back on.
It is important to follow the recommended times and speeds when using stand mixer attachments, and if your mixer feels hot to the touch when using for an extended period of time, to let it cool completely.
Were you making something steamy?
When mixing something steamy, the steam can condense on the circuit board in the mixer causing an interruption in the flow of the electricity and causing the motor to shut off. Once the steam has dissipated, the unit should turn on again (about 3 minutes).