I would replace the large filter cap also, it may have developed a high ESR. or be leaking internally, not helping the FET in it's quest for longevity.
Sometimes the FET will just decide it's had enough, but most often it gets help in the form of a bad input capacitor, mains surge, etc. I would have a really good look at the insulating washer/thermal pad under the FET for any signs of pinholes or arcing/flash-over. Sometimes the FET can blast a small piece of metal into the thermal pad when they go bang.
Exo has a good point also, one of the high value resistors in the circuit may have opened under stress, with no physical signs. For some reason, and I have no idea why, they like to blow a piece of themselves off on the PCB side where you can't always see it. Of the times I have seen this, only once was there a trace at lower potential that the resistor had arced to, causing the failure.