I had this yesterday and it was all due to a mis-configuration in my "/etc/network/interfaces" file. Make sure you are using the actual NIC/Wireless names (type "ip link" in a terminal to find them) to make the bridge and not eth0 / eth1 unless you've made the necessary changes to call them that make and make sure you have the necessary gateway/broadcast etc. in the same file if using static rather than DHCP.
Also, comment out any line in reference to your current NIC/Wireless (add a # to the beginning of the line(s)) as you should only set the IP on the bridge and not the NIC/Wireless directly.
I'm still new to the Ubuntu and Networking but this was what I learned yesterday whilst trying to add a router function to my (dedicated) server.
Good luck and please let me know if this works or not - I don't like to give unreliable feedback / help.