Unfortunately (and quite stupidly), I had .txt files set to open with Libreoffice Writer by default, and when I clicked on my sources.list file to add something, it opened up there. I suspect this is likely the cause of my problem, as now I can't get my repos to update, or install any packages.
When I try to update the repos, I get the following error message:
E: Type 'deb' is not known on line 1 in source list /etc/apt/sources.list E: The list of sources could not be read.
Line 1 in sources.list reads:
deb cdrom:[elementary OS 5.1 hera - stable amd64 (20200814)]/ bionic contrib main non-free
Commenting out the first line doesn't help either. If I put a "#" before "deb" I get the following error message:
E: Malformed line 1 in source list /etc/apt/sources.list (type)
Given that I can't install any packages or update my repo until I fix this, what do I do?