The problem was because of the plugin named “ITheme Security”. When plugin was enabled, the wp-admin panel did not work and gave the following error:
404 page not Found Error.
On disabling this plugin, admin panel worked fine. But disabling this plugin was not the permanent solution as we needed this plugin for better security reasons.
Solution: First, access the database for WordPress site. Once logged into the database, look for the wp_options table.
Select the itsec_hide_beckand option_name from the table wp_option. Replace value for its corresponding option_value from enabled to disable as shown in the image below: