ReadyNAS RND4000 v3 - Power on (fan noise), top row of LED flickers, no display, no network presence

Try removing all the disks (labeling by slot).  Then power up the NAS.

Normally it will start, and display a "no disks" status on the LCD.  If you don't see anything (and likely you won't), then the chassis has failed.  There is no point in contacting Netgear support - they won't repair hardware problems, and the NAS is out of warranty.

If it is the power supply, it is possible to jury-rig it - using an off-the shelf ATX supply.  You would need use an extension cable, since it won't fit into the NAS.  You'll need to make a small change to that cable though.  We can give you details if you are comfortable with that path.

You can also get a replacement OS-6 NAS.  With support from Netgear, you can temporarily install your current array, so you can copy off the files. I think there is a charge for this. https://kb.netgear.com/29876/ReadyNAS-Migrating-disks-from-RAIDiator-4-1-or-RAIDiator-5-3-to-ReadyNAS-OS-6

If you can connect the disks to a Windows PC (either with SATA or USB adapter/docks) it is also possible to access the files using RAID recovery software ( https://www.r-studio.com/ )

Purchasing a used NAS is possible, though you do need to keep in mind that anything you find will be at least 9 years old.  Some folks here have purchased used NAS and found they did not function.  If you go this route, make sure you purchase a v1 (4.1.x firmware), and not a v2 (5.3.x firmware).  I realize you think you have a v3 - but what you actually have is version 3 of a v1.  It is confusing. 

A v2 NAS will say "ReadyNAS NV+ v2" on the front panel - unlike yours, which just has "ReadyNAS NV+"