Do you have this entry on your firewall rules page in web admin?:
Protocol = tcp
Source Port = 80 to 0
Destination Port = 0
Destination IP = 0
Rule Type = core_input
Limit to IP = <blank>
Add it if it isn't there then use you external IP address to access web admin as before.
For reference : If you don't have a static IP address you can also use Linuxmce to manage this so even if your IP address changes you can still access your system. look at the instructions under the 'Dynamic IP Addresses' section of
http://wiki.linuxmce.org/index.php/Outside_Access