Check the power supply. Make sure the microwave is firmly plugged into a working electrical outlet and that your wall outlet is active if your home uses outlets controlled by a wall switch. Check your home's fuses or circuit breakers to ensure they have not been tripped. Without power the microwave will not respond to any input.
Make sure the parental lock is inactive. Some ovens, like the one shown in the image above, feature a keypad lock to prevent children playing with the controls. Most keypad locks can be cancelled by holding down a button or combination of buttons for a few seconds.
Make sure the door is firmly closed. Some microwave ovens will not allow any control panel input if the door is open.
Remove the microwave from the power, wait 10 seconds, then plug it back in. This will reset the microwave oven if there is a temporary problem with the software.
Press the "Stop/Cancel" button several times to cancel the current operation and return the microwave to standby mode. Re-enter your cooking instructions using the keypad as usual.