Take a small rubber mallet and tap the key. This is assuming the car is in park (automatic only) or manual transmission. The key should come out without issue. This should be enough to loosen up the parts on the lock cylinder that are binding. You may want to replace the lock cylinder at this point to ensure this doesn't happen again.
The ignition lock cylinder is a common failure item on 2000-2004 Ford Focus models. Assuming that your key is not chewed up, go to your local Ford dealer and order a new lock cylinder - you'll need to give them the Vehicle Identification Number so they can key it to your car. This should cost no more than $90. Installing it should take no more than one hour of your time if you take your time and have a couple screw drivers (phillips and slot).