I recently had a similar issue with my 14 sierra slt (5.3) ... truck had zero power. Completely unresponsive... i eventually got it to start and somewhat brushed it off. It happened about a week later...
Since i recently bought the truck, it was still under warranty by the (gmc) dealer... they decided they would send a tech to come look at the truck at the house.. before they towed it.
The tech couldnt figure it out, but after doing some research the night before i shimmied the negative ground cable on the battery. It had power instantly.
They replaced the negative ground, and so far so good... a bit concerning though. (hope its not a sign of things to come)