I strongly suspect the -12V output circuit of the ATX PSU is damaged then.
Just because the -12V is only rated at 500mA doesn't mean your regulator will only provide 500mA, what you have essentially done is short the high current +12V output to the low current -12V output. Because of their relative ratings, the -12V output would have been lifted to several volts positive and damaged the output components.
Do this:
Follow the -12V wire back to the PSU, usually it is the blue wire.
On the PSU board you will find an electrolytic capacitor with it's negative side connected to the wire. Replace that capacitor.
The next part depends on the PSU design and it varies from one model to another - usually, there will then be an inductor and another electrolytic capacitor. Change that capacitor also. There will also be a diode with it's cathode (banded) end connected to the transformer and the anode end to those capacitors you just changed. Replace that diode.
There is a good chance it will then work again but don't repeat the same mistake!
Brian.