Rule Management cannot open connection - An error has occured. Please check your logs (4339486)

Rule management dashboard is loading forever then finally error message appeared "An error has occurred. Please check your logs".

The following message is displayed in the FMS log:

YYYY-MM-DD HH:MM:SS.SSS ERROR [cwxhxbykzz:1008913240147019082284816] com.quest.wcf.core.module.function.FunctionManager - Error executing script id=system:administration_rulesnotifications_rulemanagement.getRules name="'Get Rules' from the System module Administration/Rules & Notifications/Rule Management" message: com.quest.nitro.service.sl.interfaces.scripting.ScriptingException: com.quest.nitro.service.sl.interfaces.scripting.ScriptAbortException: com.quest.wcf.data.DataSourceException: org.hibernate.exception.GenericJDBCException: Cannot open connection