Theft system light stays on and the car will not start?

This means the car is in theft mode. I would disconnect the battery overnight to see if that resets the system. This guide can help as well, can I ask if you have the original or spare key?

https://www.2carpros.com/articles/how-to-reset-a-security-system

You can try a hard reboot which is done by removing the negative side of the battery, with the cable loose from the negative side of the battery, hold it to the positive terminal for 1 minute this will discharge the memory capacitors in the BCM and PCM allowing the system to reboot. Please let us know what happens.