2003 Ford Ranger theft light flashing

Maybe a little better explaination.
1. Does the theft light still flash after the key is inserted
2. Does it flash fast when the key is turned.
If either of these is true then you should probably call a locksmith who can reprogram the key to the truck.
Otherwise I would look at grounds to the block. Take a tester and check for continuity from any point on the block to the - on the battery.
If the truck was dead for a while it is possible the Key code for the Anti theft was erased.