Checkout page is not working for not logged in users

Here is the problem.
I have installed woocommerce. Everything was working fine but suddenly it has stopped working properly for not logged in users.
The website is https://www.onlinethela.com

When user adds any product into the bag, it gets added. He gets the cart page properly. But when user clicks on checkout , a black screen comes in with asking for logins. Instead it should give the option to register the new user.

Any idea , how can we restore it ? Or make it work. It’s very critical.

https://www.ads-software.com/extend/plugins/woocommerce/