It's entirely possible that something has come up in their life, and they're having a hard time contacting you. Have you seen them active somewhere else, when they could have been talking to you?
I'm no stranger to long distance relationships. Constant contact can be hard to maintain when life gets in the way. So it's best not to jump to assumptions too fast... That being said
If it's been weeks, it's completely reasonable to start assuming things. Send them some sort of contact that's hard for them to ignore, and tell them how you feel. Tell them where you stand. Tell them you're worried, and that it's hard to hang on like this. Don't expect the worst, but be ready for it. Set yourself a pillow so that in the case that you do fall, it's cushioned.
Best of luck to you in all this. I'll keep my fingers crossed that things work out best for you