Went to start car thumb slipped off key and all power went away. No lights nothing? It did the same thing last week but came back with full power.

Resetting the Passlock system should fix it. If it doesn't you need an auto electrician or an auto locksmith.

https://www.google.com/search?q=2001+gmc+yukon+security+system+reset

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Hi I am a tech mechanic I deal with things like that Evey day lol but sounds like the anti theft or the ignition switch n lock n key to but you also have broke wires touching even in thecolom

run the fault codes first to see if there are any codes, next is to use the scanner to reset the ECM program
Lastly is to check the wiring circuit to ensure that you have not by-passed a control that stops the battery from going flat.
Lastly have a load test done on the battery because the gauge shows full voltage , batteries can fail under a load such as starting.

You have a faulty ignition switch relay. It's responsible for using a low voltage signal (key in start position) to send full battery power to your starting circuit.

Sounds like a bad ground. When it wont start run a jumper cable from the negative side of the battery to a good ground on the motor and try the body also. Rule that out first.

Check your battery connections - where the cables connect to the battery posts AND where the cables connect to the starter/main power feed and where the ground cable connects to the engine and/or frame. A poor or corroded connection can cause intermittent problems and poor charging performance.

Welcome to FIxYa.com

Check for a drain remove the positive side battery connect a test light clamp end to the battery wire and the pointed end to battery post light will be on. Pull one fuse at a time. When test light :light goes out that is the circuit draining the battery

Regards, Lee Davidian

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Sounds like an antitheft problem I would start with the antitheft system. Usually when you only have one key all keys have to be programmed together even the original. When you have more than one it is easy to program additional keys.

You nailed the problem in the second sentence. Likely the belt tensioner is failing, causing the belt to become loose and slip. That in turn allowed your alternator to slip instead of charging your battery. While you drove, you were running on battery only. There is enough power in there to drive for sometimes over an hour. When you went to re-start, there was insufficient power to operate starter (which needs more juice to turn than anything else in the car) This lowered battery voltage further, tripping the security system. You will need a new serpentine belt, idler pulley and battery charge, then re-activate security system. Never worry about giving too many details...far better than some posts I read that would have said something like "my car. click why" And fifteen questions later I sometimes get close to the answers you provided...nice to see someone out there that's literate!!!
good luck with it!

glue guy the starter is probably bad..id replace it

michael buck what year and engine do you have