NV+ RND4450

Step one is to see if it's the chassis or the drives causing the problem.  With power off, remove the drives, marking the slot that came from.  Then power on.  If it posers on and reports "No Disks" an RAIDar can see it, it's probably not the hardware (though it could still be in the SATA sub-system). A nesxt step would be to put a single spare dribve in slot one and see if the NAS powers on and formats/installs the OS on the drive.  If it does, then power down and move to slot 2 and power up.  Then repeat with 3 and 4.

If it passes that the hardware tests, check each drive with manufacturer's tools (such as Seatools for Seagate) and see if there is a single bad drive.  If there is just one bad, and you were using XRAID or another redundent RAID mode, return the remining drives to the chassis and see if it boots.

If the chassis and all the drives come up good, you may need an OS re-install.

If none of these work, come back and we''l see where else you can go.  Make notes of any messages you see on the NAS display as you go through the steps.