To make an ethernet card work, there needs to be a kernel module ( driver ) loaded that supports the particular ethernet card you have. This is true for any kernel. Drivers are compiled at the same time the kernel itself gets compiled.
You are running Ubuntu. Odds are you have the driver, you need to find out which one, for your card and use the modprobe command to load it.
First thing is to open a command prompt, and run the command /sbin/lspci. That command will list the devices connected to your pci bus. In the list you should see the ehternet card. It will look something like this :
00:07.0 Bridge: nVidia Corporation MCP61 Ethernet (rev a2)
Look for the key word, 'Ethernet'. The rest will probably be different on your system due to hardware differences.
You can then google for 'linux driver for your ethernet card' Usually common cards are not hard to find. If you can not find the driver, post the output from the command, and someone here can help.
You can look at the output of the command 'lsmod' to look at the list of loaded drivers. If the correct module is there, you can try to activate the device; with the command 'ifconfig eth0 up'. That command will not work if you do not have the driver loaded.
Once you know the name of the driver, as root, or sudo, try to load the driver. The command will be 'modprobe nameofdriver'. All commands without the quotes, just what is between the quotes.
If you get no response, that is success. You should be able to see the module listed with the command /sbin/lsmod.
Once the driver is loaded, and you use the command 'ifconfig eth0 up', you can look at Network Manager to see your configuration, and modify it if necessary.