You don't say what door it is, but either way you'll probably have to try and get the door panel off. This can be real difficult when the door won't open, but you may not have much of a choice. If it's a rear door you may have the child-safety on, which you may be able to unlock and then open the door from the inside.
This all takes some expertise and may best be done by a repair shop.
Here are some in your area if you chose that route: http://repairpal.com/directory?address=76309&car_brand_names=Ford
had the same problem many times with the right rear door. would not open no matter how much banging and pulling then after giving up it would later work. Then when it happed again, I found that by just pressing the power door locks on and off fast until the door would open worked. since then i spray the latch with wd40 every month and have not had another problem.