Please do not mark your posts as urgent - it does not get more attention when you do that.
What distribution are you running? Most distros have network configuration tools to help with this sort of thing. If you want to do it manually, have a look in /etc/hosts. It may be useful to run the following to see if your old address is in multiple files (substitute your existing IP address below):
sudo grep -r 'your.old.ip.address' /etc/*