Locks in park and now won’t shut off
I hope, before posting, you called for a tow.
Very odd you can not turn the key to off when transmission is in park.
Let us know the diagnosis,
Do you mean you cannot move the transmission from P, and are also unable to turn the key from “on” to “off”? That sort of problem often means the ignition switch has to be removed for a look-see. Look in your owner’s manual to see if there’s an inter-lock over-ride, under the dash, etc, you can press to get the lock to behave enough you can turn the key to off.
I would hope after 12 hours they found someone to solve this or the car ran out of fuel.
I would just pull the ignition and/or fuel pump fuse, then disconnect the battery, grab a large bourbon a couple of aspirin. And call for a tow in the morning. OTOH a 1999 Grand Marquis probably isn’t worth the tow and repair charges.