ReadyNAS RN4220 wont boot up after shutdown from WebUI

I agree that this sounds like it's failing POST and that a hardware failure has likely occurred.   I do not know the proper baud rate for the 4220 console connection.  When I had a 4220 to troubleshoot, I inserted a video card in place of the 10GbE card.

Unlike its predecessor, the 4200, the 4220 does not use a standard motherboard -- it's Netgear unique, so troubleshooting is difficult because Netgear has provided nothing to start from.  One possibility is the RAM.  It's standard DDR3 1600 ECC -- very common for servers of that generation and pretty cheap these days.  You can try with just a single RAM module in the white slot closest to the CPU and see if it then boots, trying each module.  The CPU is another possibility, though less likely.  A power supply is highly unlikely given the dual supplies -- it only needs one, the other is a backup.  But you can easily try each by itself. 

I don't remember the flash on that one being a DOM as on the later units, so not user-replaceable. But on most ReadyNAS, the flash is unpowered except for in boot, so failure is unlikely.  I don't know about the 4220.  But with the network LED flashing, maybe that is a sign it's looking for an OS.  Does your router show the unit is connected?

If your NAS is set up with multiple volumes and none exceeds 8 drives, then  a smaller unit is a possibility for data recovery.  You can insert the drives from each volume separately since the OS is duplicated on all drives.  There are a lot more options if none exceeds 6 drives. Otherwise, you'll need a 12-bay unit.  You may be hard pressed to find a 12-bay unit down under and shipping from the US would be a lot.  But perhaps you can find a 3220 (same mobo, different processor) or 4220 that somebody is willing to part out to reduce the shipping and risk that the problem is in the mobo.  But your best bet is likely a standard 12-bay server on which you can run Linux and use that for data recovery.