Tell me your running your own dhcp server or something like that. If not then you must be running a router of some sort. Did you define any IPs to machines? fileserver or something of that nature that you just booted up? If your just connected right into your cable/dsl modem (bad idea but what ever) then call your ISP and be like "Somethings messed" and tell them to fix it because that shouldnt happen. Ill check this post again to see if I can help further.