It can be your connection. Make sure you are connected to the internet. If you are, and you can't join, try lowering the quality of everything in the options menu. If none of those work, you might need a new router if you want to join that server,
I know you are trying to help, but for future reference, quality options will not affect your ability to join servers, it will only make it so that when you spawn, you lag out (if your computer is that bad).
Anyway, see if you can join any other servers with a --- ping, as from my experience, this doesn't always mean that the server is unjoinable, and from what I know, different pings don't show for different people, so if it says ---, it must say --- for everyone else.