ReadyNAS RNDP6000-Parts? Wont power on.

The usual suspect for a 6-bay legacy ReadyNAS not turning on is the power supply.  It's a commercial SFX supply for small form factor PCs with a longer 24-pin cable and more 4-pin MOLEX connectors than most, so it wasn't designed to last as long as most folks keep a ReadyNAS.  The +5VSB (+5 Volt StandBy) power is the most susceptible, since it's always on (even when the NAS and fans are off) so as to power the power-on and WoL circuits.  If you also don't see the Ethernet LED lit when it's connected to your switch/router, then that's the likely cause.

While I used to recommend a more do-it-yourself approach with a standard SFX supply and extenders/adapters, the seller on eBay that has some already modified has lowered the price and I've seen enough testaments here that they are a good supply, I now suggest that as the easiest solution, though I have never purchased one.

If you have a "spare" ATX12V supply on hand, you can connect it externally just to see that that is the issue and/or to power it long enough to move your data to a new NAS.  If you do that and run it for a while to recover data, it's best to use some masking tape to cover the gap where the cables go into the NAS and the old power supply fan area so as to get good air circulation over the drives from just the main fan.

As for the display, it may just be the cable that connects it.  I had one that had become a bit mangled and the display was affected.  It uses the same 5V that much of the other circuitry uses, so that wasn't an early sign of the other issue, as bad 5V power would affect a lot more.