This has happened to me and it's basically broken me. My friendship was 7 years of real lovely times and closeness and he was my best friend, the fact he's a bloke makes it 100x more complicated as I am second-guessing myself (Is it because his girlfriend is jealous? Am I secretly in love with him and didn't know?) but the overall sentiment is the same.
This happened to me once before although the person then was an ex and it took me years and years to get over, for me it was not the loss of the other person - that is part of life sometimes, friendships do end and evolve over time and we have to learn to let go - but the fact that someone I thought would NEVER ghost me had ghosted me.
It's the grief of a friendship ending (which is real and those who are hurting right now will feel me on this) coupled with the lack of understanding and closure that is searingly painful. I keep wondering what I did or said, or why if he wasn't keen on being friends any longer he couldn't have simply told me so I could move on with my life.
Sadly there is very little you can do, I haven't heard from my best friend for three months where before our contact was pretty regular although definitely not daily and often not every week. It hurts massively when someone you thought would always be in your life drops off the face of the earth and you have no clue why. It hurts when you are worried about them and don't know if they are okay. But eventually you have to take actions at face value, and if someone is acting like they give not one shit, well then they probably don't give a shit. 
I sometimes think I'm too intolerant with my partners and lack compromise, but when it comes to my friendships I put up with too much crap and flakiness. I'm taking this as an opportunity to set boundaries in my current and future friendships so I never allow myself to be taken for granted so much again. So I suppose it's been a useful lesson, but a very painful one.
Best of luck, OP 