Sorry about the late reply, just seen this question.
This is a common problem on most Ford models, the tumblers inside the lock barrel wear out so it won't unlock the door, you should still be able to lock the car.
You should take your car to a garage that can take out the barrel and replace the tumblers, I've done loads of them.
If they tell you that all your locks need replacing then they're trying to rip you off, or they don't know how to do the job.
If you're that concerned take it to a Ford main dealer, bearing in mind you'll pay main dealer labour prices.