The chances are, if you have left it long than X amount of days, the 'case' will be closed (probably in favour of the complaining party), because you didn't respond within a reasonable amount of time.

If this is the case, you will need to email PayPal normally (not through the dispute resolution section) and see what they can do for you. PayPal are usually pretty strict and don't bend the rules too often, however, so I doubt you would have much luck in getting a reply from them this way. Your best bet would be to call them and explain the situation - tell them that you received no notification of a complaint whatsoever.

Of course, if this person didn't even open a dispute, and simply charged back at his card issuer, I'm not sure PayPal could do anything about it. The only thing I could suggest in this case is that you do not provide the services to this individual, or if you already have, you raise a complaint with PayPal telling them that this guy has perhaps fiddled you - after all, if he's simply done this chargeback so he can receive free services, he should be banned from PayPal, there is no question about it.

Either way, if you had already provided the services, and there is no evidence that this guy is lying, you will probably just have to accept the fact that you aren't getting your money back. You will need to move on, forget about it, and pay your money back to PayPal (otherwise you risk having your account frozen / terminated and they may add additional fee's to your account).

EDIT: If it's an open issue then simply submit your reply, explain your side of the story, and wait to see what happens. You cannot do any more than that. If you don't see anywhere to submit your reply, i'd suggest emailing / phoning PayPal and letting them know. Don't put it off, else the issue will be resolved in favour of the other party, and you will lose out.