Wordpress: How to switch to the default theme if wp-admin is not accessible.

If you have mysql database(phpmyadmin) access you can switch to default theme by the following method

Step 1: Login to the phpmyadmin account
Step 2: Open 'wp_option' table
Step 3: Find these values in 'option_name' fields   `template`, `stylesheet` and `current_theme`, All these fields are set to the current theme name which you have activated.
Step 4: Now you have to change these fields values. change their 'option_value' to the default theme. Set 'default' or any other theme name like twentythirteen, twentyfifteen etc on there 'option_value' and refresh your site.