Why can't my Xbox 360 access the internet?

I have an old Xbox 360 without a wireless adapter. So it gets connected to the internet by a bridge i have set up on my computer. I run an Ethernet cable from my computer to my Xbox and have the connections bridged. It worked just fine until about a week ago. It just won't connect. the tests say that it can't get a ip address and this has happened before but i fixed it by going to my router settings and clicked an alert and unblocked it. I've rebuilt the bridge and reset it multiple times and now i'm out of ideas. Please include multiple ideas in case I've already tried the first. I have a Cisco router if that helps.