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.

Is the transmission, if automatic, upshifting properly and into the highest gear? If not, that'll absolutely kill your gas mileage! Is the check engine light on? HTH. -Jim