I assume your server runs on linux.

Stop your server.
Open your ts3server_startscript.sh (e.g. with WinSCP) and make sure that line 6 looks like this:
Code:
COMMANDLINE_PARAMETERS="${2}" #add any command line parameters you want to pass here
Save and close it.
Then start your server (e.g. with PuTTY) with the followong command parameter and replace "PASSWORD" with your new password:
Code:
./ts3server_minimal_runscript.sh serveradmin_password=PASSWORD
You now changed the password of your serveradmin query.
Stop the server again and start it with the normal ts3server_startscript.sh again.
Login with YaTQA as serveradmin with your new password.
Select your server, go to the advanced tab and change host message mode.
You can now connect to your server with the client again.

Reminder: always know your serveradmin query password, which normally gets created and shown in the console after the first start of a TeamSpeak server