This article is for cPanel Accounts. Determine where your account is with this guide.
If you happen to be stuck in a situation where you need to reset the WordPress theme to the default one, follow the next these easy steps.:
Navigate to cPanel >> phpMyAdmin
Open the console
Select the Desired Database in the SQL Tab, select each of the below lines individually and execute each of them
as in the image below
update wp_options set option_value = 'default' where option_name = 'template'
update wp_options set option_value = 'default' where option_name ='stylesheet'