Well actually the software centre is an application that runs from your hard
drive. It is the software that is on the internet. The software its
contained in repositories which are websites the software centre accesses.
It could be that you don't have the particular ones in your software sources
list that you need, or that one or more are temporarily offline.

Try refreshing your list. I'm assuming your web browser and email are
working fine. Sorry I can't be more precise but I use synaptic on Kubuntu,
which its a little different.