Hi. I have a 2005 Hummer H2 , went to the store, came out turn the key nothing, passlock light went out,Battery is full, towed it home sat for two days turn the key to on then to accessory can hear a slow thunk thunk thunk, key wont turn off, key is stuck in accessory position and and cant shift out of park with key in the on position and battery is now dead.
so what a bad ignition switch?

 

Bad ignition switch, maybe a bad ground somewhere, maybe a bad battery.

 

Start with the basic, take your battery to an auto parts store and have charged/tested for free, be prepared to buy a new battery.

 

does your cluster actually show you are in park?
reason i ask is because the shift cable bushing on the transmission end goes bad leaving the transmission in whatever gear it popped off at. not in park no key. another probable cause can be the shifter wiring in the center console. the wires tend to break from moving the shifter also causing to not release the key. both very common issues with the H2.

 

Ive learned with gm when the battery is dead you cant get the key out so there's that. Battery is good.

 

Battery age & test results?

 

I had a similar situation, except I could remove key.

Turned out my starter was toast. Replaced and cranks/starts no problem.