You are the only person who can try to evaluate what happened and then once you do, you have several options of what to do next. There are times when couples can lose interest in the other person, the reasons can be many and varied, But what matters is that if it happens, you know how to act to take care of your emotional health.
Now here comes the hardest part, what should you do next? Do you call him first to see what the situation is? Well, this is up to you. Do you feel like you need answers to move on? Or can you accept the fact that he has treated you badly and let that be reason enough?
Whatever your own personal reasons for "ghosting" you, you must ask yourself if this is how you should be treated. Is this the kind of behavior you deserve? Unless he has strong reasons to believe there is more to it than the simple explanation that he doesn't love you enough anymore, I would advise you not to waste any more time with him. Sometimes this can be difficult to accept, but you really have to trust that it is for the best.
What to do
All the great love stories didn't start with Romeo abandoning Juliet, or Mr. Darcy hiding Elizabeth Bennett until he had a chat. The best relationships come from a point of mutual investment and mutual attraction. Everyone deserves to have someone who is prepared to invest in them and not drop them when they want to. Also, if they've done it once, who's telling you they won't do it again in the future?
But I understand that everyone is different and has different opinions on what is correct, so if you still want to contact him for whatever reason, make sure you do it the right way.
Text him or give him a quick call, but if he doesn't answer your calls or texts the first time, don't let yourself fall into the trap of becoming the nagging, annoying girl who keeps texting and refuses to move on.
What if it reappears?
It is possible that weeks, months or even years later it will appear again and you will try to contact him again. Now it is totally up to you what to do if this happens. You may have moved on and are no longer interested, or maybe you're still curious why it disappeared in the first place. You may not know the reason behind him suddenly avoiding you, but do you really need a reason?
They stopped trying to contact you, which basically means they stopped caring about how you were, and when someone stops caring, it's usually because they are no longer interested in others. The sooner you realize this, the happier you will be in the long run. Just ask yourself, why is he trying to talk to you now after so long no contact? Find out what has changed. I was prepared to lose you without any kind of explanation, apology or reason. Is that really the type of person you want to have a future with?