A metal-oxide varistor is a discrete electronic component whose electrical resistance decreases rapidly as the voltage across it increases. Placed in parallel with the circuit it protects, it remains highly resistive during normal operation but becomes highly conductive during a transient voltage surge, shunting the excess current safely away. They degrade slightly with every surge they absorb and eventually fail, often dramatically.
The surge protector stopped working because the primary MOV sacrificed itself during that lightning strike.
MOVs are sacrificial components. If you are repairing a device that took a power surge, checking the MOV for physical damage or a dead short is usually the first step.
Related terms
Varistor
A voltage-dependent resistor that protects circuits by shunting excess current during a power surge.
Resettable fuse
A passive component that restricts current during a fault and automatically restores conductivity when the fault is removed.
Crowbarred
Protected by or triggered into a state where a circuit intentionally shorts its own power supply.