EDIT: You maybe able to follow the advice in this linuxquestions thread and uncomment the relevant entries in your
sources.list and try to run
Debian 7 reached EOL as of May 31, 2018. Those repositories are no longer active. You can still receive support for Wheezy but it is going to be a lot less painful and expensive on your part to make moves to migrate to Jessie or even Stretch.
On the Debian Wiki there is advice on what to do now that Debian Wheezy is EOL. You may also want to check out this entry to get more information on what to do.
If you are upgrading to Jessie, your
sources.list needs to changed to the following:
deb http://deb.debian.org/debian/ jessie main contrib non-free
deb-src http://deb.debian.org/debian/ jessie main contrib non-free
deb http://security.debian.org/ jessie/updates main contrib non-free
deb-src http://security.debian.org/ jessie/updates main contrib non-free
deb http://deb.debian.org/debian/ jessie-updates main contrib non-free
deb-src http://deb.debian.org/debian/ jessie-updates main contrib non-free
apt install apt -t jessie,
apt upgrade, and finally
apt-get dist-upgrade to update and upgrade to Jessie.
If you absolutely need to stay on Wheezy and cannot change to Jessie, either due to issues outlined here or because of some other limitations on your environment then you may need to change over to the archive mirrors or look into paid ELTS support.
Archive mirrors will look something like this:
deb http://archive.debian.org/debian/ wheezy main contrib non-free