I don't know the exact circuit, but it sounds like the feedback device has failed.
Power supplies like that often have a transformer with a primary and two secondaries. The transformer operates in flyback mode, where during one phase of operation, the current increases in the primary and there is no current in the secondaries, as it is blocked with diodes. In the other phase of operation, there is no current in the primary and the stored energy goes into the secondaries.
The two secondary voltages are in approximate proportion to their number of turns.
On secondary feeds the main 5 V output and the other supplies the control chip on the high-voltage side.
There is a feedback, so when 5 V is reached, that is signaled through and opto-isolator, and the control chip turns off.
If there is no feedback, the control chip keeps on running and the output voltage gets too large, which is what i think happened to your power supply.
In that state, the supply to the control chip may be too large as well due to the other secondary now having a larger voltage. A lot of control chips will detect that and turn off, which is hy the voltage isn't huge.