Users starting to fail login

[DanCF]

This has happened occasionally, but it is starting to be more frequent, so I decided to post here. I did a search but I couldn't find anything.

Clients which could once login, are no longer able to login. (not all at once, but this has happened to a few servers). Even if I reset their passwords, and try it immediately after, I still get login failed. I have tried deleting the user, then recreating a user with a password, and it still fails.

[WolfStar76]

When the user fails their login, what's the error message in the lower portion of the screen?

For example, if they're failing the name/password challenge it'll say
Bad Login (name and/or password wrong)

If, on the other hand, it says something like:
No reply from server
Maybe the server is offline
or maybe teamspeak is not running on it
that indicates that it isn't the user accounts that are bad, but rather that people are unable to connect to your server. In which case you'd want to check your IP hasn't changed, and/or that your port forwarding and firewall settings haven't changed.