Leave a comment on the question for those people. If they actually do visit the site, they'll see a notification of a new comment. If they care at all, they'll respond, either answering your question or confirming that they solved the problem a different way. If that's the case, kindly ask them to post an answer detailing their solution and accept it when the system will let them do so, informing them that this is necessary to mark their question as "resolved".
Otherwise, don't worry so much about it! They're here to beg you for help. If they don't want to help themselves, then I can't see how there is any way that you can help them, either.
Remember that participation on this site is completely voluntary. There's no way of cajoling users into participating more actively. If you see a question that you think has been completely abandoned and poses no redeemable benefit for future visitors to the site, please flag it for moderator attention.