@Enver - I do know about encoding options in text editors and that's usually when I screw this up, but what's weird is I have had Notepad++ open for a really long time with other css files from other sites AND this site, but all are ok and open back up properly. Go figure.
As for your question...
"Also, have you tried rendering the contents of the file as text in a simple page of HTML (marked-up with the UTF-8 charset, of course)? If it is readable to the human eye, then I would copy and paste the content from the page into the original file and resave it with the UTF-8 option."
I'm not sure what you mean by that. It sounds like you say to save it as .html with UTF-8, open it in a browser, then copy and past into a text editor and save as UTF-8? That didn't sound like it would do anything, but I tried anyhow and it didn't do anything. Maybe I mis-interpretted what you said?
ALSO, the css file has a "@media" at the beginning of it, could that make a difference? This is a client's template I did not code from scratch.
I actually thought the same thing, but here's what actually happened...
I had the file open in Notepad ++ editing, then uploading to the server. A few hours later I went back to edit the file again, but for some reason decided to download the most recent file from the server because I thought I may have edited the one on my computer and then decided not to go with the changes (I do that sometimes). So I wanted the exact file from the server. At this point, I usually backup the one on my computer (call it backup_delete_later or something) and then download the one from the server. But I just skipped the backup not thinking (probably because I've done this procedure countless times without error) and downloaded and over-wrote it. Opened it up and it was now all on one line.
So, it's either the editor saved it that way or somewhere between the editing and uploading that messed up. What's weird is that I had other css files open and editing as well and they're all fine! That's why I'm stumped and want to figure out what I did wrong so I don't do it again. But more importantly, I not have a messed up file that will take me a very long time to sift through for changes, or redo all of the changes I made again (not fun because there were lots of changes sitewide so catching them all would be a pain).
My solution right now if I don't hear anything better, is to use an editor that will create line breaks after every "}" so at least I can compare an older backup with the new code.