That's a tough one. I think it might come down to how the project was structured and communicated. That is, they may have specifically set it up so that you could be getting things done when they went off on a long vacation (or whatever), expecting progress to have been made by the time they got back, but maybe forgot to fill you in before they left that that was their plan. Or maybe they got hit by a bus.
Depending on how much you know about the client, you might find other ways to attempt to contact them (e.g., phone number associated with skype account, web site contact, domain registration info).
Otherwise, you seem to have done the responsible thing by pausing. If there was a miscommunication, the client should be understanding of the subsequent delay. If it was something more severe on their end, your professionalism will likely be appreciated whatever the circumstance, too.
I might even be inclined to continue with some of the work in the same circumstance, though. My reasoning would be this: if there is some part of the project that I would find useful outside the scope of the client, I would do it. Then, if the client does still want it, everything is ready in a timely manner. If they don't and I have to refund their money, I now have that bit of code for my own use in other projects/apps/frameworks/etc.
Thanks for the replies. I've tried his skype and email. I never got a phone number from him. I did find him on social media and could ask his friends about his whereabouts and whyabouts, but there's a fine line between being dutifully diligent and breaking privacy boundaries.
I think my best plan of action is to finish what can be done and stay true to the contract, and take a chance that if I need to refund any payment, at least I will have gained more programming experience.
Thanks for the advice.