I had a friend that moved very far away. We stayed in touch for a while. But he was very busy running a business and raising a special needs toddler with his wife so contact became sporadic. At first I was very understanding and would repeatedly initiate contact that generally, but not always, seemed to well received. After a year or two, I pretty much gave up except for an occasional outreach. I thought that he must be mad at me about something though I couldn't imagine what. After a year or so of minimal contact, I learned that he was suffering from, and hiding, crippling depression. I found out because he hanged himself.
If you care for the person, put in the fucking effort. You don't know what's going on in their life. I will never forgive myself for not trying harder.