Help. :( Security light Cheyenne 1500 98 S.O.S

Computer problem
1985 GMC C1500 V8 Two Wheel Drive Automatic

Greetings, I searched through every website in Spanish and I have not achieved with a precise answer, my problem is that the change the ignition cylinder from the pik-up model Cheyenne 98 years, it flashes a light on the dashboard "Security" and turns off the car, grateful for any support, thanks