Your cache contains a history of all visited sites, images, audio files, etc. When returning to a site, files are retrieved from your cache instead of downloading again from the web site. This is meant for faster internet browsing, but a very full browser cache can actually cause problems and may not reflect the current web site.
If you can access an electronic resource in a private or protected browser window, and you cannot access the same resource in a regular browser window, then your browser cache may be interfering with access and need to be cleared.
Chrome: Clear Browsing Data
Firefox: How to Clear the Firefox Cache