How can I fix my piano key that makes no sound?

I play the piano a lot but I don't misuse the keys. I was playing it
today and my "A" was working perfectly. Then a couple hours later, I
was playing a song that actually has a lot of A in it (if you know
what I mean) ,and it wasn't working! Its not working later today
either. Do I need a professional to come fix it? The hammer moves, but
makes no sound! It doesn't get all the way there. What should I do?

Again, it my piano is relatively new, and this has never happened! it
was working perfectly!