Also, is the network slow internally (to your server) or externally or both?
I'd suspect your router anyway. Consumer grade routers are never seem to last forever. You can test a few ways: plug a PC directly into the cable modem (or what ever network modem) and see if your external speed comes back. This wont solve your internal network speed if you are having issues (obviously without a router).
You could use various tools, like wireshark, ping...
You could disable your firewall on all machines. Maybe an update has caused issues with your network?