Flip open the the bottom charcoal filter vent to find the small list of error codes. This is usually saved for service technicians, but it can help you define the error code that displays on your microwave.
When you try to start the microwave, it will flash off, beep multiple times and then the "F7" will flash on the screen. The microwave will shut off after 30 seconds of displaying the error code.
The "F7" error code indicates that the 1,100 watt inverter has failed inside the microwave. This means that food cannot be cooked until the inverter is replaced.
Check with Sears customer service to see if there is still a warranty available on the microwave. If there is, you could receive service and replacement parts free of charge.
The inverter and the installation could cost hundreds of dollars. It may cheaper to replace the microwave if there is no warranty or protection on the appliance.