I guess this just triggered me to remember that there are a couple of other friendships I’ve “lost” (in addition to the one mentioned in my previous reply) but really moreso from being on your friend’s side. I’ve fought wicked depression my entire life, stemming from trauma in childhood. Basically it’s a struggle to see myself as someone who is valuable enough to be someone’s friend and my inclination is to see myself as someone who is tolerated and endured by people and friends rather than someone that enriches people’s lives and someone people want to be friends with. It’s illogical, but is a constant emotional narrative despite me logically knowing better. The truth is that when I step back and logically think through it, I know I’m freaking awesome and an incredible friend who puts himself out to help the people I love and whose advice is (inexplicably!) sought out by my friends. I fight my emotional brain and fight to keep the friendships I have and to never be needy or a “burden” on my friends, but there have been a couple of times in the past where I’ve become convinced that I’m awful and responded by just completely disappearing, like resetting a hard drive, moving on, and starting over. I don’t know what your friend is going through, but you may not have done anything at all. It could absolutely just be something your friend is going through privately and doesn’t know how to share with his friends or is afraid to, so it could be that it’s easier to just disappear entirely.
Other than just time and distance I have only ever really lost one good friend. Except I still don’t know what I did. He just stopped returning our calls. We aren’t sure precisely when he disappeared but wee know we invited him to come over for the OSU/Michigan game and he never responded and we never heard from him again. I still wish we knew what happened.