Antitheft System light flashing and I cannot turn the key?

So, the ignition key will not turn at all, or it runs, and does nothing? Or is the security light flashing and so the truck will not start? If it is the latter, here is a guide so you can reset the system:

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

Let us know what happens and please upload pictures or videos of the problem.