ford fusion won't start anti theft flashing

You've already identified your problem; the anti theft is active. That has to be addressed or the car will never start. The car needs to be programmed to accept the key. If it's the original master key then the car may have lost it's recognition of the key. Some owner's manuals have a procedure to help your car relearn or resync with it's key, you'll have to look for that info in your manual, and if you don't have a manual look for one online. Otherwise you may need to take it to a dealer or locksmith for programming.