Possible reasons include shorted rectifier diode(s) and open output transistor(s) allowing the power supply voltage to rise past the voltage limit of the capacitor.

What I thinmk you might want to do is to remove the transistors and diodes and check them out of circuit. If you have a schematic, it will help you find the parts that are most likely to have failed. But even without a schematic, you can check the transistors and diodes. Be sure to remove them from the circuit when testing.