If you only have one key, then the computer must have all the key codes erased from the computer and the surviving key and at least one new key will have to be reprogrammed. If you have at least two working keys in your possession, it is relatively easy to add more.
To add keys, I always tell my customers to get in the vehicle, buckle their seatbelt, and then put either one of two working keys in the ignition and turn it to the RUN position, wait for the THEFT light to go out, then turn the key off and set it aside. Repeat with the second working key. Put the new key in, turn it to RUN, wait for the THEFT light to go out, turn the key off, and then the vehicle should start. This process works with 99% of all Ford vehicles with the PATS system.