How do I fix the compression fitting connection on my refrigerator water line?
This video shows basic assembly of small compression fittings. The only things I would add:


the ferrule (whether plastic or brass) is indeed a "single use" item, as mentioned by @Tyson and @Ed Beal
you can use a brass ferrule on plastic tubing, but not a plastic ferrule on copper tubing
the video says to tighten a single turn past hand tight. I say tighten well, regardless of turns needed; you need to actually squeeze that ferrule onto the tubing.