In your error.log, you have an error message such as:
java.sql.SQLException: Unable to get a connection from pool:com.day.cq.resource.pool.OutOfConnectionsException: Maximum number of Connections exceeded 5: no free connection left
Increase the maxconnections parameter in /bootstrap/systemdbpool.xml. Restart your instance after the change.
This issue occurs when you need a connection, but all the connections from your connection pool are being used.