I'm not gonna lie, when the object of your affection suddenly vanishes from your life, it can really be hard to cope with. You just have so many questions! Where did they go? Did I creep them out (refer to figure 1.0 below)? Was the messy bun I was wearing last week not messy enough? Did they somehow see me eat that sandwich off the floor that one time? Ughhhh!
Okay, now please try to get a hold of yourself. Whether this person knew how you feel/felt about them, it's probably not your fault that they left. For all you know, they might have a super legitimate reason for leaving. Like, maybe their aunt died of a terrible illness and they had to leave town to say goodbye to her and deal with her will and stuff. And is it really any of your business? No, not really. But I get that it still sucks and you're sad/guilty/angry. So, I am giving you permission to convince yourself that you have done all that you could to get them to stay. Even if you didn't. Tell yourself whatever you have to to make yourself feel better. That's probably terrible advice, but missing someone is probably one of the most painful things to deal with in the world so I honestly can't judge someone for denying/justifying their actions if it makes them feel better about it.
However, you should remember that it's okay to feel sad about it! You really liked this person! So it's important that you do nice things for yourself. I mean, get a few of your closest friends and invite them to get drunk in the privacy of your own home (you will embarrass yourself if you do this in public) and just let the emotions flow. Laugh at memories, cry, yell and bash this person for breaking your heart by their unexplained absence, talk about his penis, whatever! And then your friends will inevitably tell you that you deserve better, and you know what? They're right! Trust me, there will be other babes that will come into your life and maybe they too will leave eventually, or maybe you will leave them, or maybe the relationship works out! You could have an entire revolving door of crushes if you wanted to! So listen to your friends, there is a good reason you keep them around!
So now you should maybe do actual nice things for yourself, as opposed to things that will destroy your liver. I'm talking about classic "breakup" behaviour. You know, getting your hair done, going to the spa, buying a weapon. Become super healthy. Like, join a gym and work out almost every day. You will feel so much better about yourself and added bonus (!) if you run into this person someday your body will be banging and they will hate themselves! You pretty much can't lose in that situation!
You also need to accept the fact that this person is gone. I mean, chances are your life isn't a romantic comedy and this is just the part where you've or they've screwed up in the relationship and now you have to win each other over again. No team of writers wrote a speech for you to passionately deliver to this person in order to convince them to love you. A grand and romantic gesture that's begging for them to come back seems like a good idea in a movie, but in real life it's just creepy as fuck. So I wouldn't recommend doing it. However, if your life is in fact a romantic comedy, then by all means go for it cause you knew it was gonna work out in the end an hour ago.
Lastly, in the strange event that this person does turn up in your life again, be prepared for it. I mean, don't spend all of your nights dreaming of this moment and what you'll say, but rather, think about how you want to look to this person. Do you want to give them the satisfaction that they deeply hurt you and you're bitter over it? Or do you want to be friendly and cool and give the impression that you're mature enough to have moved on? Yeah... this is a tough decision.
And if none of this has helped you out, just be grateful that for a while, you won't have to deal with this awkward situation.