That's the high temperature limiting device on the upper thermostat. If the heat goes too high, it cuts the power. Maybe the T-Stat is getting sticky and has trouble releasing, or maybe its just a fluke and popped off from vibration or something.
If the stat is old and becoming sticky, then this will probably happen again within the next month. If it does, then this reset switch is the first thing you should check. And then the thermostat should be replaced also because of the age and non-reliability.