A common cause is security plugins which have been configured to prevent access to files within the WordPress uploads folder. If you have any security plugins running, please review their settings to ensure that access to the uploads folder is permitted. A quick way to test whether a particular plugin is blocking access is to temporarily disable it and refresh the page.
If the files don't appear to be blocked by a security plugin, I suggest contacting your web host to ask them to whether they are able to identify the server configuration issue which is preventing access to the files.
If you have, or suspect you're having this issue, and aren't able to solve it with the above steps, please contact me or leave a comment below.