The security light is flashing and my car won't start after getting my instrument cluster serviced.

If you have a vehicle with the passive anti-theft system (PATS) which is usually a Ford/Lincoln/Mercury then something triggered it to turn on. When this happens, the security light on the instrument cluster will be flashing and the vehicle may not start.
The immobilizer system detects a de-syncing issue. This can happen from a low 12V vehicle battery charge, a replacement unit that has a different VIN or immobilizer code, or a key fob that has a faulty battery.
 
Resetting the PATS (Passive Anti-Theft System) can vary from model to model but the concept is the same. If you're not certain you can find out online what the reset procedure is for your specific make and model. If no information is available you can do a hard battery reset.
 
How do you reset the Ford Ranger anti-theft system?   
Simply turn the car on and off nine times, leaving the key in the on position. If you hear the doors lock, you should be all set and try to start the vehicle.