2000 Tundra- no power, possibly anti theft issue

My daughter has a 2000 Tundra Access Cab and last night she was stranded because there is no power to the truck, the horn goes off if you touch the door handle, hood or anything. I found a reset trick to put it in valet mode but I can’t even do it because there is no power. I believe this may have been triggered by the key fob battery dying just one day prior. We have unhooked the battery twice for different lengths of time and rehooked it, and when power comes on it’s very temporary, not even long enough to turn the key to the on position. Here’s a pic of the fob in case it matters

 

Yep. Looks like you have an aftermarket alarm in there. They tend to flake out after 10-15 years. The alarm you have is similar to what @RUSTYNUTS and a couple others have.

That said, define “no power”, have you checked the square and the flat fuses in the fuse box next to the battery?

 

Oh, and nobody did anything stupid, like connecting the battery backwards, right? Sorry, gotta ask, it’s happened before, we’ve seen it several times.

 

There are no lights on inside at all, like it’s dead. One fuse for heater to be replaced and we’re going to get a fuse tester now

 

If I'm looking at an online diagram correctly, you're going to want to focus on the fuses with windows, it looks like the 120A (big rectangle at the top of my box here) is sending most of the power for core function and some into the cab. Those are also (often) a freaking bitch to pull out. And I think also the pink square "50A" 2nd from the bottom of my box, it's the main feed to the interior fusebox, I think.

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Updated my reply with a little more info. I missed one of the fuses in the list.