A diode in that configuration is only good for protecting against reverse voltage if there is a fuse in front it (as it is supposed to present as a short), or you have a current limited power supply that can't make it go 'pop' and let the magic smoke out... as a short-circuit is the typical failure state of diodes, not open... so you don't want them to fail when protecting something!
As fire219 pointed out, those regulators are early in the circuit - one providing 5v to the rest of the system, and the other providing 5v to the USB ports. So hopefully they took the lot and blew out... otherwise, you can pretty much write that board off... as next up would be the main RK808 power management IC... and if that got fried also... the CPU and peripherals are toast.
There are four components likely to be decreased in the first instance... 1) polarised capacitor C2001 2) diode D2000 3 & 4) SY8113Bs U2000 and U2001 -can't tell which is which from a quick glance at the board. If you were an electronics tinker, you could put a current limited 5v supply on the GPIO 5v and ground pins... as it seems they are attached to the VCC5V0_SYS... so it should at least boot if it's going to? You'd want to get rid of U2000 and U2001 first in case they're shorting anything, and are dead anyway as they've let out the magic smoke.
btw, if you had anything on the PCIe bus, it's probably toast as well, as the reason the rockpro64 needs 12v is that PCIe has 12v...