Originally Posted by GreyStang
Were you parked on a hill?
Try shifting gears with the engine off...
^^perfect reason to use parking brake!^^
If you did park on a hill, it puts a load on your transmission if you didn't use your hill brake. Therefore you are going to have to move the car forward, or backward depending on if you parked uphill or downhill, to be able to disengage your brake. I did this once and it was the only way I could get it out of gear.
If you didn't park on a hill, maybe you snapped a band or something? I don't know much about auto trannies.