I am presently experiencing symptoms as described here:
https://bugs.launchpad.net/ubuntu/+source/apt/+bug/493154
Example:
127-12:42 djh@noneedto ~$ aptitude search chrome
aptitude: error while loading shared libraries: libapt-pkg-libc6.10-6.so.4.8: cannot open shared object file: No such file or directory
127-12:46 djh@noneedto ~$ apt-get -h
The program 'apt-get' is currently not installed. You can install it by typing:
sudo apt-get install apt
You will have to enable the component called 'main'
So, my thinking, as describe in the bug, is just to download the dpkg for apt. Unfortunately, this is not an easy thing to Google, because usually when people are looking to know about "apt" and "download" and "install" it is "how to use apt-get" ... but wher do I get apt from if it is missing in the first place?
I am running Ubuntu 10.04.3 LTS (lucid)
Update: per mailq's answer, the fix here was:
wget http://archive.ubuntu.com/ubuntu/pool/main/a/apt/apt_0.7.9ubuntu17.3_amd64.deb
sudo dpkg -i apt_0.7.9ubuntu17.3_amd64.deb
If you are fixing this problem, cruise over to http://archive.ubuntu.com/ubuntu/pool/main/a/apt/ and select a package suitable to your system.