If he didn't read your message, he likely didn't read messages from others. If his account is gone, it sounds like he deleted it like so many people are getting fed up with social media.

What could’ve happened?

Anything. Could he be dead? Unlikely, but anything is possible.

Did he win the lottery and go into hiding to avoid people begging for money? Even less likely but also possible.

Anything in between? More likely and also possible.

Myself, I quit Facebook a long time ago, but when trying to recover my account to save my old pictures the website required me to upload a photo of my driver's license for account verification. I said screw that and have let my account sit idle ever since. I'm not sure if it will get deleted after so much time, but my mom used to tell me how a bunch of people were still posting birthday wishes on my old page long after I stopped using it.

I would recommend looking into other ways to contact him. You could also do some digging and look up their name to see if you find them on any public records. If they really did pass on you would likely find an obituary online, or possibly they are in jail (also public records). I would think they just quit social media, like in my case, and you might need to search through common acquaintances.

Probably sick of social media. I have a few friend requests/message requests I never bothered accepting since I'm preparing to delete those social media accounts anyway.

Maybe he is working for the FBI like the hollies said..

Can you track down a phone number or address? Sending a letter is nice if you can!

Which platform? My grandma just called me that she can't reach me, because I finally got rid of WhatsApp. She asked how to contact me now, I felt a bit sorry.

I did something like this a few years ago. Not saving someone’s life, but not reading messages and deleting my Facebook account.

I just got overloaded on it all and was going through a breakup at the time and didn’t want to keep talking to anyone. Nothing personal against anyone, I was just over it all and wanted out of Facebook, out of the addiction I felt to constantly check, and out of the privacy concerns of Facebook.

Could be a similar reason?

Also, was this Facebook or something else? Because if it was Facebook, it’s possible a privacy setting prevented the message from successfully going through.

There was this girl who was the daughter of a popular figure I knew many years ago and we lost contact over the years. I tried reaching out to her once on there and she never read the message. She was very social so I know she would have read it if she got it. But there is or was a privacy setting that could hide messages depending on how Facebook viewed a message and relationship of the two accounts. In our case, we had no mutual friends and no contact before on that account of mine and no other commonalities so Facebook hides the message in some deep place most people don’t check because it’s treated like spam or an unwanted stalker. I found some messages from random people there a few times on my account too. So maybe that happened here too?

Of course this is all assuming this is Facebook and they still have these measures in place all these years later.