Originally Posted by GDC-SRT
The shifter does this for electrical trouble, usually.
Three weeks sitting could lower battery power, especially if alarm was in the on position.
You might try and charge battery, and reset the system by disconnecting it.
Pizzaguy could be onto something with the brake interlock.
Of the threads I read, it is usually something electrical.
The TCM, like mentioned before, a bad sensor plate, the battery or like Pizza mentioned, others have also had the issue of the Brake pedal switch.
One last thought --- have you check the tranny fluid? It's a sealed system but there are also threads on here about how to go about doing it.
Eitherway, use the tool for now. Don't force it to do something it doesn't want to do.