404 Error. Very Frustrated operator!!

Can someone please help? I have installed and posted one post with a video, 2 days later I tried to post a new post, alas every time I attempt to navigate to the new post page I am sent to the error page. I am a single user and the administrator. The blog is being hosted with my website. I am using “cutline-3-column-split-11” theme if that makes any difference. I didn’t have this trouble with a previous blog using the default theme, although I can’t see that this is a theme issue.Could it be some form of corrupted script to do with user authorization? Regardless all I need to know is how to fix it. Thank you