Some computers don't let the car shift if it's veeery cold. Dunno if that's your problem, but does it go away once it's all warmed up?
Also, did your shift lever bend.. or anything weird like that? If it was in "3" instead of "OD" or whatever that would cause that (does on my Buick at least).
Get a buddy who knows electronics to test the PRNDL switch to make sure it's in the right spot. Could've shorted out internally or broke... if the computer saw the switch saying you were in "3", that's how it'd drive.
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