mssql on ubuntu will not start, cannot update 404
Fixed by guessing this was due to a server update and the source url changed. I added the current source per their instructions https://docs.microsoft.com/en-us/sql/linux/quickstart-install-connect-ubuntu

This added the NEW source with a sudo add-apt-repository ... per the docs. I then manually comment-out (or you can remove) the old 404 source in /etc/apt/sources.list.

Finally sudo apt-get update -y ... and start the server and I can again see it running (netstat or whatever to see the service up).

To be clear the URL changed:

from 
https://packages.microsoft.com/ubuntu/16.04/mssql-server 

to https://packages.microsoft.com/ubuntu/16.04/mssql-server-2017

Comment: adding '-2017' to sources, or changing sources for existing installations is highly likely to both break them and makes these sources look dated, possibly all goals driving the change. That sucks.