If they have already wired you a deposit + some additional, my first thought is that they are in the middle of some unexpected emergency that has become their sole priority at the moment.

Buyers flaking out after making a verbal commitment but before any funds change hands happens all the time, but for someone to go "dark" AFTER parting with some of their money suggests there may be some significant extenuating circumstances. (I know for me, there's no way in hell I'm parting with ANY of my money until I am 100% certain of the purchase!)

If it were me, I think I'd try to tactfully reach out. I would frame my questions in the most nonthreatening way possible, remind the buyer that you are good to go whenever they want to make final arrangements and pay the remaining balance owed, and try to approach the communication from a position of concern that "everything is ok" on their end.

I definitely DON'T think this is one of those situations where you would want to get aggressive, accusatory, or put forth any ultimatums (you might be kicking a man who is already down - the LAST thing I'd want to do to someone, personally). The fact that you have been paid (some) is probably a good indication that this is less likely to be a scam or any other malignant behavior, so (if you can) I'd give the benefit of the doubt on this one and wait a while longer to hear back before declaring the deal "dead" and looking for another buyer.