Why would a rooster stop crowing?

Roosters crow due to a biological signal which sends signals to the brain. It is the same type of signal that helps a human body return to a normal temperature 1 hour before waking up or water lillies to close their leaves before the sun rises. The problem you are experiencing with your rooster may be a result of this signal no longer firing. It could be an age thing, sickness or disorientation. Also, if a rooster is not sleeping well throughout the day, exhaustion could be a factor. Lastly, scientists believe that crowing is a way roosters mark territory (like dogs urinating on everything). If there is no perceived competition to protect its hens, coop or territory - it may neglect to crow at all. Sorry I couldn't be more specific, but your question didn't have any supporting context or facts.