Contact your ISP and address the issue with them, they may have some troubleshooting tips for you. Otherwise there is always the option that I would normally take which is unplug your modem and leave your house, walk up to the first person you find and chuck it at their face. . But seriously, try setting a password on your modem/router. Some actually do have an allotted connection limit (since you said everybody else was connected to it as well).