DIAGNOSIS You've likely tripped the thermal overload protection. The tool gets hot from extended heavy use, especially in dense material, and shuts itself down to prevent damage to the motor.
FIX First, let the tool cool down completely for 15-20 minutes. Before you restart, make sure you're using a sharp, appropriate blade for the material. When you go back to it, use a lighter touch and avoid forcing it—let the tool do the work. For long cuts, plan for short breaks to let it breathe.
IF THAT DIDN'T WORK If it still won't run after cooling, the problem might be in the trigger switch. Dust and debris can get inside and block the contacts. You can try blowing it out with compressed air, but if the switch itself is faulty, it'll need to be replaced at a service center.
HEADS UP This tool isn't really built for marathon plunge cuts into thick hardwood. If you're doing a lot of that, you're better off stepping up to the corded model (DWE315) or the more powerful DeWalt 20V MAX XR oscillating tool.