what's caused my mileage to drastically drop from 28 mpg to 15 mpg?

Have you changed your driving or added weight inside the vehicle

When is the last time you did a complete tune-up on the car ???? are your under inflated ???

Check the air filter first. That's the most common cause.

Does the car have a lot of miles on it? If so it may be worn out but I would check the catalytic converter to see if it is plugged up. Like joemom says do a tuneup and check the air in your tires but those would probably only account for a 10% or so drop in mileage.

Also add some fuel injector cleaner to gas tank and clean throttle body with throttle body cleaner.