My 06 HHR 2LT has been sitting for a few years, and I thought the battery had just died...but looks like bigger issues.

I replaced the battery tonight after climbing in the trunk and using the emergency release, and it sprang back to life. All the lights and such in accessory mode were working, got a few indicators on the dash (check engine light and check airbag message).
However, I went to start it, and then it died again. Everything turned off, even in accessory mode, and the key was stuck (guessing Passlock). I manually removed the key, but now I can't even get things on in accessory mode again, all I have is the blinking orange padlock on the far left. Hoping its something silly, and she's not dead dead.