Incorrect Database Login Credentials
Your WordPress site uses separate login information to connect to its MySQL database.
Corrupted Database
With so many moving parts with themes, plugins, and users constantly deleting and installing them, sometimes databases get corrupted. This can be due to a missing or individually corrupted table, or perhaps some information was deleted by accident.
Corrupt Files in Your WordPress Installation
This can even happen sometimes due to hackers.
Issues With Your Database Server
Several things could be wrong on the web host’s end, such as overloading the database from a traffic spike or being unresponsive from too many concurrent connections. This is common with shared hosts as they utilize the same resources for many users on the same servers.
Spike in Traffic
Depending on the web host you use, your server might be unable to handle many concurrent database connections. An increase in traffic can lead to temporary problems with the database.