Mostly in this situation you can access the WordPress admin area, the blog’s main page, but when opening a single post, your WordPress posts returning 404 Error. First it must be ascertained that the 404 error is being caused by the web server or by WordPress.
Check static file
Open your static file(e.g. a jpg image) like:
If it is does not exist in media uploads directory, you have found the source of your 404 Error. If it is Continue Reading ...