You didn't really specify what version of Ubuntu you're running (especially since you said you've been running it for a few years), so I'll just assume you've been upgrading regularly and we're talking Ubuntu 12.04 here.
You need to modify the /etc/network/interfaces file...
$ sudo gedit /etc/network/interfaces
...with the following (assuming your network interface is eth0... change the values as appropriate to your own situation):
iface eth0 inet dhcp
Before saving the file, make sure there are no other references to that interface (in this case, "eth0") anywhere (there shouldn't be). But if you'd set up static manually in the past, you might see something like this:
iface eth0 inet static
If you don't see that block, you may want to just give that a shot and see if your interface comes up with a DHCP-assigned IP address when you restart networking (sudo /etc/init.d/networking restart).
Whatever you decide (DHCP or static), you'll need to restart networking after editing that file:
$ sudo /etc/init.d/networking restart
If you set a static IP, however, you also need to make sure that the other information that you would normally receive via DHCP is correct as well. This includes your DNS information (such as nameservers). Edit the /etc/resolv.conf file:
$ sudo gedit /etc/resolv.conf
This file includes your nameservers (you may also include a domain to search, but not necessary). It's usually just your router, but you may be using something else entirely (I'll give you OpenDNS here, just because I know it works):
If you don't have any nameservers specified, then you won't be able to use domain names for Internet access (i.e., "ping google.com").
You can test DNS by performing a simple lookup:
$ nslookup google.com
You should get something back that includes something like:
If that works, then you're all set!