OnTheEdgeIn07

We have an 07 Edge and the brakes are really hard to depress. Seems like the power part of power brakes is not working. Started with a hissing sound under the dash and progressed quickly to becomming hard to depress. Is there an air assist type of function that could go out? And would the air sound/hissing sound be a symptom that anyone could use to suggest where I should look first?