Problem with apt-get update

I just did a fresh install and when trying to run sudo apt-get update i get the error that 3 files are missing.

sources.list
sources.list.d
apt.conf.d

How can I fix this? I found some orientations for other distros but would they work for this one?

What can have caused the issue? I did a install with this same ubs stick (I keep it to launch from USB when I can't access my laptop, so it was unchanged) and worked fine first time, now I re installed it and it broke.

Thanks for the help