Hissing sound, no brake fluid, brake drops to floor help!?

I'm having brake troubles. I noticed a hissing sound coming from the brake pedal a day or so ago. Then I noticed that when I put pressure on the brake pedal (to stop obviously) the pedal drops to the floor. The car eventually comes to a stop but takes a long distance and time and is really scary. Got home ok and checked the brake fluid and there wasn't any in there. My mechanic two weeks ago said my fluid was low so he filled it for me. What could be going on here? Obviously I have a leak somewhere, but what could be making that hissing noise? Also, how much do you ballpark it will be to fix?