RNDU6000

All it takes for ReadyNAS to be displayed is the 5V rail of the supply, so a lot can be wrong.  The first thing I'd check is that nothing came unseated (like the RAM module) in transit.  The power supply is the most prevalent first failure on the 6-bay legacy NAS, especially so if it is power cycled a lot.  They tend to last a while, but many people keep them for longer than they will last.  It will normally quit on a power cycle, so also moving it may just be a coincedence or be one of the few times you have power cycled it at all.

The supply is a standard SFX format supply except the connector wire length.  The one it came with is rated 300W, but it is better to go with a 400W replacement to insure you have enough 12V power, as the 12V on the original Seasonic (which is no longer available) is higher than a typical 300W supply.  I've not really found a new vendor I'm really happy with in terms of spec vs. price.  Most will not have the required three 4-pin Molex connectors, so you'll need to move the connectors from the original (they are easy to move) or buy a SATA to Molex adapter or Molex splitter.  You also will need an extender for the 24-pin ATX connector (typically 5", but depends on the make of the supply).  The Apevia SFX-AP400W seems a good choice, though I have not actually used one.  Many have used the Corsair SF450, which is nicely modular and likely very hardy but it costs twice as much.  There is a vendor on eBay selling a replacement, which appears to be an already modified Apevia.  I have seen some supposed SFX supplies with the fan or input power connector in the wrong place, so do look at the old one and the new to be sure you are getting a standard SFX.  Note that the fan does not need to protrude, and some have used ones where it is not centered or is slightly larger (like with the Corsair) with no apparent issues.

If you have a standard ATX12V (Current standard -- ATX with 24 pin main and 4-pin CPU) supply, or can "borrow" one from a PC, you can use that externally to see if that's the problem or even power it long enough to move data to another NAS.  If you do plan to run  it for any period of time, just do what you can to keep the proper air flow, especially over the drives.  Sealing the gaps and original power supply outlet with making tape is normally good enough unless you are running it in a very hot environment.