The microwave/convection ovens can generate a lot of heat which is supposed to vent outside. But sometimes people either forget to open the vent, or don't realize it must be open. If closed, the oven can overheat. To prevent fire, the ovens typically have a thermal fuse that opens, causing the oven to lose all power. And even if your vent is open, you could still have a thermal fuse that needs replacement. If you're able to find a service manual online, you may be able to repair it yourself for just a few dollars. Or you may be able to bring in an appliance repair person to fix it for you.