The main fuse or the thermal cutout is probably blown. If the main fuse is blown, most likely the high voltage capacitor is shorted and must be replaced. If this is the case, it's a good idea to also replace the high voltage diode because it took a large over-current hit before the fuse blew. Be sure the oven is unplugged before measuring the resistance of the fuse or capacitor (the diode should read open in both directions because the forward voltage is more than a typical ohmmeter can put out). (Note: replacing the fuse without identifying and fixing the short or overload will simply waste the new fuse. It's not like the old farmhouse with one fuse for a dozen outlets going out because one item too many was plugged in.) The capacitor is available on Amazon for less than $17 (be sure the voltage and uF values match the one in your oven), the diode is about half of that, and the fuse should be under $3.

If the thermal cutout is blown, check the magnetron cooling provisions - make sure the fan turns freely, the blades are clean, and all vents are clean. After replacing the thermal cutout, verify that magnetron cooling fan runs at speed when the oven is running. Keep your hands and tools out of the oven if you are testing it with the cover off. The high voltage side can be energized by as much as 1800 Volts. An arc from this can cause serious damage or personal injury.
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