Whirlpool window unit a/c power comes on but no air comes out

My A/C unit is 4 years old and we use it very infrequently each summer, maybe about 5-10 times. It was working fine and then all of a sudden it stopped blowing air. When you turn it on the power comes on but all you get is a humming sound which gets louder and softer. It is under a 5 yr. warranty for only certain problems. How would I determine what the problem is before having to call in a service technician? Thanks!