First of all, try connecting to MySQL through the command line to check if the MySQL server is working and the user password is correct.
mysql -u USERNAME -p PASSWORD
If you can successfully connect to the server, go to the phpmyadmin configuration file and change the connection string configuration.
See this: http://docs.phpmyadmin.net/en/latest/setup.html
But if you cannot connect to the MySQL server, your problem is your server, not phpmyadmin. Therefore, go to the xampp control panel and configure the server configuration file.
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