2000 mustang gt. The theft light is on and wont

You'll probably have to let the dealer re-program the theft deterrent processor

When the car is off, a slowly flashing anti-theft light is normal. When the key is in one of the on positions, that light should go off. If it thinks it detects a theft condition, the light will flash quite rapidly and prevent starting. You can also check the mileage readings in the odometer display. If you see dashes instead of numbers, that is a clue that some part of the onboard computer has gone wacky.

You already tried one of the steps that I would recommend (unlocking the door, though that may not be enough by itself. Here are a couple of things to try.

First, pray that this isn't the anti-theft computer gone awry. That is uber expensive to replace. Following that prayer ...

Verify that the battery is fully charged and showing about 12.8 volts with the headlights off and about 12 volts with headlights on. If the voltages are low, try jumping the battery to see if you can get it to start. (A low battery can occasionally cause an anit-theft fault.)

If you have another key, try that one. The chip in the first key may have gone bad. If that doesn't work ...

Disconnect the battery negative cable for 5 minutes. Reconnect, and lock and unlock the doors several times with the remote. Check to see if the anti-theft light is still flashing rapidly. If not, try starting.

Anything more than these steps will need to be handled by a qualified shop. (For example, a wire in the steering column could be damaged by the tilt mechanism, or the computer malfunctioning).

Good luck.