Once you’ve spun yarn, can it unravel? Is there anything you need to do to make sure that doesn’t happen?

I don’t spin but I have had yarn come untwisted when I’m working with it.

Sure, any yarn can unravel. If you’ve knitted you know that spinning helps it maintain the twist, but it’s possible with certain ways of knitting or with a very loosely spun yarn that it can come untwisted. I rarely have it happen though.