It will be better if you consult to your respective hosting provider. They might help you on this. And also you can follow this steps, this may help you.
1> Check wp-config file:
It may occur when there is a mis-match between your wp-config.php. First of all, make sure you (or the WordPress install) has correctly set up the database name, database user name, database password, and database host. See this link: http://codex.wordpress.org/Editing_wp-config.php
2> Make changes on wp-users:
Then you can access phpMyAdmin (most hosting providers offer this in their control panel), Then go to the wp_users table and find the row for your user_login[wp-admin login ] . Note the ID of this row. Now, go to the wp_usermeta table and find the wp_capabilities row for your user_id . Set the value of this row to