I did the same thing and I made a thread about it. In my case it blew the protection diodes in the HDD. It shut the psu down instantly. Anyway… with everything unplugged from the PSU, just plug it into power mains, short the green wire to the black wire on the molex connector and it should fire up. Those things have over current and over voltage protection to save the PSU and prevent a major event. So depending on what you had plugged into the wrong cable, it can stop the computer from working.
https://www.badcaps.net/forum/showthread.php?t=58320