Check the server list at top right to make sure you're selecting the correct server.

I chose the right server.

If you chose the right server, and you're using a third party login like Google / Facebook, you need to make sure you're logging in the same way as before (through third party rather than direct email input).

Past that, if you still can't find the right server on the account, you need to make a CS ticket via the ? icon at the top right of your client.