Almost Every A/T, from every manufacture hides a shift-release button (to over-ride the ignition, brake, and shifter moutned button).
It's there when you need it.
Like steviej told you. It's very hard to duplicate such problems. When you try, and go through the system, more often than not, it's user error. Like forgetting to push the brake, or forgetting to have the ignition turned to the ON position.
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To those who have replied, thank you. First, the car was not parked on an incline and yes, the parking brake was on. To the rest of you, if and when it happens again, I will step on the brake and try to shift.