|Well, I'm unable to help troubleshoot this much further without actually being on site to do so hands-on. |
If all clients suddenly stop communicating then the only thing it really could be is the piece of equipment they're all plugged in to. The chances of 15 computers NIC's all breaking at the exact same moment, or, 15 network cables all stopping working at the same moment, range somewhere from slim to nonexistent.
You will likely need to call a professional in to help you figure out what's going on. If it were me I would try a third switch before doing anything else but if you don't have access to yet another switch, call a consultant.