You overloaded it. The motor has a thermal cut-out to protect itself from overheating.
Unplug the blender. Allow the motor to cool for 20-45 minutes. Do not attempt to restart it until it has cooled.
For nut butters or oil-based foods: Do not process for more than one minute after the mixture starts to circulate. Processing longer causes overheating.
If that didn't work and it consistently stops quickly, you might be using incorrect settings. Do not process recipes at lower or higher speeds than recommended, or for longer than recommended. Use the tamper for thick mixtures, but do not use it for more than 30 consecutive seconds.
Heads up: If the problem persists after cooling and using correct techniques, contact Vitamix Customer Service.