I'm working with magento 2 .There I faced problem with one time checkout so i attached a image with this anyone can help me.
Welcome to the Community forums.
In order to try to help you should show us an error. Maybe you can start by enabling Magento's logs and the take a look to those files to see if there is a clue on those messages.
Also, you should check your browser console (sometimes you can get infomration about the ajax call)