Microwave circuits need to be 15 amps (not watts). For this, you will need
at least 14 gauge wire.
That said, it is unlikely that the outlet stopped working. It is more likely
that the circuit breaker tripped--have you checked that? A lot of older
homes do not have dedicated circuits for the microwave. It doesn't take a
lot more than a coffeemaker or kettle in addition to the micro to get that
circuit to trip--especially if the original is only 15 amps.
If the micro has a dedicated circuit and the breaker tripped, you may have
something wrong with the wiring leading to the outlet or with the microwave
itself as a microwave alone should not trip a 15 amp (or larger) circuit.
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