I’m not sure how old you are, but you sound young. I’m guessing that what you meant by basically living together is that you were always over at his place, but you two never had any kind of real or serious conversations about your relationship, and you never made a real plan to move in with him, it just sort of ended up that you were always over there, right?
I’m guessing that this fellow was enjoying himself with you, but then kind of got freaked out or spooked because the relationship was moving too fast (since you guys never discussed your relationship it just sort of morphed). Unfortunately, some young guys (and probably some very immature men) do this type of thing. They just leave and you never hear from them again, and if you get the chance to ask them about it, later, they will tend to blame you for making assumptions or for being desperate or not seeing things as they really were, instead of just talking to you and telling you the real reason, which could have been one or several.
Most likely he enjoyed the sex with you for awhile, and now he’s moved on to someone else. Or he had another girlfriend at the same time (maybe in a different town) that didn’t know about you. Or an old girlfriend came back into his life and threatened him if he didn’t dump you immediately. Or some of his male friends told him that he was being an idiot for letting a girl move in with him.
But like Auggie said, are you sure he isn’t sick, injured or dead? Do you know for a fact that he is still around, somewhere? If you do know he’s fine and alive you have a couple of choices (neither of which involve getting back together with him).
You can simply walk away and don’t try to contact him. Or you can write him a VERY POLITE letter explaining that you were worried and embarrassed that he dropped off the face of the planet without so much as a simple explanation and you would like to know why he did this instead of just talking to you (but don’t expect that he will tell you, and do expect that he will say mean things to you like I suggested above).
The reason you should make sure your note (if you choose to write one) is kept super-polite is because he is going to quote you, either on Facebook or to his friends, or to other people that you know.
Either way, if you write him a note, or choose to walk away, you are likely to run into him again sometime down the line (usually in the most un-expected or awkward situation) and you should have a very calm “act” for what you will say or do when this happens. Don’t set yourself up to cause a scene.
I’m so sorry that this happened to you. It really sucks : (
Next time you start dating someone, go a little slower and do a lot more talking about what is happening while it’s happening. And don’t let this particular situation get you down on dating or guys in general, just move forward in a positive manner. : )