There are many reasons why you are unable to fetch the data from your MySQL database. Following are some common causes for this to happen:
Incorrect database login credentials. It is possible that database login credentials are changed but not updated in the connection string of the config file.
You can open your config file, collect the password and reset it for your database user.
MySQL service is down/unresponsive when the website has too many concurrent connections or is bumped with lots of traffic.
Please restart the MySQL service to fix it.
Database table corruption. Sometimes table corruption can result in a database connection error.
Above mentioned are the common causes. However, if you face any error other than these, you can contact us to diagnose it further. Please raise a support ticket.