See here: http://www.cisco.com/c/en/us/products/collateral/unified-communications/unified-communications-manager-callmanager/prod_end-of-life_notice0900aecd807329a8.html
5.0 went end of support almost 4 years ago, so the downloads will have just been aged out.
That happens!!! Don't shoot the messenger..
What are you trying to get hold of and why?
Hello Aaron, and Brandon,
I want you to note that CUCM 2.x, 3.x, and 4.x are EOS/EOL, too; but their download sections are still available on Cisco web site, as you can see in the screenshot below:
It's strange, isn't it?
Yeah you arn't going to find anything 5.x related on Cisco's site at this point. You should still have the media discs if you need to do a reinstall. Any support for upgrades isn't going to happen in the 5.x train.