2003 LR discovery. Went to turn ignition on. Suddenly went completely dead. No power to anything. Wouldn't think battery is issue with such a sudden shut down. What should I check first?

check battery cables one is loose

Let us assume you have not run out of fuel and the fuel gauge is ok.

Normally cars do not stop suddenly if battery is the issue. Could be the alternator but you have installed a new battery. The fuel system seems to be the problem. Could be blocked fuel pump.

u have a dead short on main live wire check for overheated wires

If nothing works including the lights, something happened between the battery and the fuse box.
There are some fuseable links between the battery and the starter solenoid or between the solenoid and the fuse box.
I'd start with the battery cables and look for those fuseable link wires.

I just answered your other question about this same issue. You need to stop replacing parts long enough to DIAGNOSE the problem so you are not creating further problems by changing all the parts on the car.

I'm thinking its the battery. Heat kills batteries. You said it wont jump start. That's a dead ringer for a bad battery. It may have enough juice to power some things, but wont run the starter, or the fuel pump, or the ignition(spark). Try a new battery and let me know!

I encountered a similar problem on a chev 4cyl with a coil pack ignition ( no distributor). It would run for a while and suddenly die out and wouldn't restart until a 10 to 20 min. cooling period. It turned out to be a crankshaft position sensor going bad. On this engine it was located in the engine block under the ignition module on the firewall side.

It sounds like it could be the starter solenoid. I would remove the battery and have it tested, then when the battery is put back in make sure all battery connections are clean and tight. If you go to autozone.com, you can register your vehicle for free and have access to a full online repair manual. Good luck and hope this helps.