Best friend stopped responding to me and disappeared from social media. Advice?

Hey everyone! So as a background I (20F) had this bestfriend(21M) that I had been friends with for 4 years. As a background, we met in high-school and very briefly dated for a few months. We both decided that being friends would be better, which completely worked out. We used to talk every single day through out the day. (And that is not an exaggeration) And i would also like to add that we never hung out in person after high school, we just kept our friendship online.

Back in April we were talking as normal and suddenly he stopped responding. I figured it could’ve been a multitude of reasons. Maybe his phone had broke, he was just busy, family troubles ect. I didn’t think much at first but grew more concerned as a couple weeks had passed. This was strange, since we had never had any arguments, he seemed fine/well and it was very out of the ordinary for him not to text me. It has now been 6 months and I’ve yet to hear from him. I have him on all social media platforms and he hasn’t posted or been active on any of them since. (And I can still view his profiles so I know I’m not blocked) We always used messenger to chat and almost never used actual text message via phone number. (Which I’ve contacted using the phone number hed given me a couple years ago. Its possible that its no longer his number) I’ve also had a friend of mine search his name to see if maybe he had created new profiles but blocked me. I’m very confused and concerned. Anyone have any possible explanations? I’m at a loss:/

TLDR ///// best friend of 4 years stopped responding to me and no longer uses or is active on social media