Disconnecting the battery activates the anti-theft (PATS) on late model cars.
You didn't mention it, but what is the theft light doing? Blinking?
Try locking and unlocking the driver's door with the key. If you have a key FOB, lock and unlock with the FOB.
Turn the key on but do not crank, Listen for the fuel pump to run for 3 seconds. If you do not hear the fuel pump run, Stop and find out why.
Confirm that the theft light goes out after a 3 second prove out. If not, STOP and find out why.
Does it start now?
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.