If a girl suddenly goes quiet when you ask her if she is single does that mean something?

Answer 1

It means she is wondering whether you are a stalker or if you like her in some way, and she is probably not overly comfortable with the idea.

Answer 2

It could mean that she is not comfortable sharing that information or that she is in a relationship. It's important to respect her privacy and not push for an answer if she is not ready to share.