by 
Why has one of my sons quit speaking to me?
I do not know the context. However, it seems likely there is a lot of pain behind this behavior, and, of course, it is causing you pain, too. Most people who do this kind of thing have given up, and are feeling hopeless that anything can be different. Since they have cut off the communication, it is now all on you to try to repair the relationship. The problem with being cut off is that your son’s narrative about you is frozen in time, and furthered by the thoughts in their head. It will be important to “control the narrative” some by encouraging them to interact with the real you—but the real you may need to interact differently than you have, since they have quit speaking.
I would recommend a variety of approaches (email, text, mail, phone call, showing up on their doorstep), all with the initial message of wanting to let them know you care (without strings). Once this has been communicated, I would express once or twice (but not more, so as to avoid badgering them), your desire to hear FROM them, wanting to UNDERSTAND them. You have to be prepared to be rejected as you try most approaches, but not to be overtly angry, demanding, or dejected to your son. As the son, they have relational power. But as the parent, you have relational power, too, and, even if they are an adult, you still have a lot of power and you are still the parent. It will be important to continue to offer unconditional messages of love, care, and concern, and not to demand a response or a positive response in return. Carl Rogers, in his book On Becoming A Person, speaks about how most of us grow up with some messages of Conditional Positive Regard (e.g., “I will love you if. . .”), and that what we need to become who we were created to be are messages of Unconditional Positive Regard (e.g., “I love you. Period.”) It will be important that he experience you in the latter way.
This is a hard situation to be in. I wish you well as you work to repair it!
