I noticed that my rooster (8 months old) stopped crowing about three weeks ago. It turned out that he had Marek's disease and we had to put him out of his misery yesterday.
SO I think that not crowing could be the first sign of disease? Purely guessing here. There are probably many other reasons, but disease could be one.
I'm so sorry about your rooster. This one's not vaccinated (the majority of the flock is) so I'll keep my eyes peeled. I realize there's not much I can do once it hits.