Did this occur suddenly or has it been going on for awhile?
If it just happened, I would check the tire pressure and see if you have a dragging brake (most often that will be the rears with a snagged emergency brake). Air filter good? Fuel filters? Once saw a car get bad gas mileage after the driver hit s curb and messed up the alignment.
If this problem has been more persistent, check the spark plugs. Got a lot of weight in the car?
Check what type of gas you are using. If you accidentally put in E85 ethanol blend your mileage will head south big time.
Lastly, is your car shifting properly? You didn't give your car's mileage, but a worn out engine or tranny will impact mileage.
Good luck.