how can I solve "package build-essential has no installation candidate problem?

I am trying to install the g++ compiler on a fresh installation of ubuntu 9.10 linux. I know that I need to install the "build-essential" package first. I used the command "sudo apt-get install build-essential" but I get the following result:

craig@craig-workstation:~$sudo apt-get install build-essential
Reading package lists...Done
Building dependency tree
Reading state information...Done
Package build-essential is not available, but is referred to by another package.
This may mean that the package is missing, has been obsolete, or is only available from another source.
E:Package build-essential has no installation candidate
craig@craig-workstation:~$

Any information about how to resolve this issue will be greatly appreciated!!!

I like to say "Thank you to François Tissandier!!!! Your response, "open the "Software Sources" item in the System>Admin menu, and check what you have there ? Make sure all the internet source options are activated in the first tab." was the solution to my issue!!! Problem solved!!! Thanks again!!!