arduino nano broken or not after reversing l7805 pins?
Something has probably been destroyed. If you are lucky it is eg a shorted tantalum power supply capacitor, but it is likely that the damage is more general. A reversed regulator may act as a diode and pass the 12V through to the "output". Look for power supply filter capacitors. Removing them with correct supply voltage MAY restore operation. May :-(.

Any electrolytic capacitors at the points circled in red MAY have gone short circuit with excess voltage - usually tantalum capacitors will do this but other types usually won't.