1999 Ford expedition steering

So last night i was driving home, and suddenly the steering wheel started acting funny. it would turn left, but when i turn it right, i have to turn it half way so i can change lanes. by the time i got home, steering was impossible. it wouldnt turn left nor right. It seemed like there was a problem with the power steering. but i cant figure out what it is since there are no leaks of any kind under the car or around the engine. (this was the first time this car has been driven in the last 3 weeks).
PLEASE HELP ME! i need this car :frown: