If you can't login to Wordpress admin, or rather, you are logged in, but can't do anything, like you've lost all permissions, you have probably changed the table prefix in your wordpress config.
To remedy this, you also need to change a few rows in MySQL:
In the table usermeta
UPDATE `wordpress`.`NEWPREFIX_usermeta` SET `meta_key` = 'NEWPREFIX_capabilities' WHERE `NEWPREFIX_usermeta`.`meta_key` = 'OLDPREFIX_capabilities';
UPDATE `wordpress`.`NEWPREFIX_usermeta` SET `meta_key` = 'NEWPREFIX_user_level' WHERE `NEWPREFIX_usermeta`.`meta_key` = 'OLDPREFIX_user_level';
UPDATE `wordpress`.`NEWPREFIX_usermeta` SET `meta_key` = 'NEWPREFIX_user-settings' WHERE `NEWPREFIX_usermeta`.`meta_key` = 'OLDPREFIX_user-settings';
UPDATE `wordpress`.`NEWPREFIX_usermeta` SET `meta_key` = 'NEWPREFIX_user-settings-time' WHERE `NEWPREFIX_usermeta`.`meta_key` = 'OLDPREFIX_user-settings-time';
UPDATE `wordpress`.`NEWPREFIX_usermeta` SET `meta_key` = 'NEWPREFIX_dashboard_quick_press_last_post_id' WHERE `NEWPREFIX_usermeta`.`meta_key` = 'OLDPREFIX_dashboard_quick_press_last_post_id';
In the table options
UPDATE `wordpress`.`NEWPREFIX_options` SET option_name = 'NEWPREFIX_user_roles' WHERE option_name = 'OLDPREFIX_user_roles'
