90% of the time this is because you tried to access a members page without being logged in or you tried to access a page that you do not have permissions for based on your membership level.  Because our site is so large, we use a caching system that caches the pages you visit to load them faster for you the next time.  So you are viewing a cached version of a permission based page, that remembers this and then stops giving you access to the pages you do have access too.
The easiest way to fix this is to first logout, then clear your cookies, cache, and complete internet history on your browser.  Log back in, and your problem should be resolved.  If it isn’t then check under the members tab, and see if the member options are showing.  If it only says Login, or register under the members tab, you are not logged in.   If you see members home, and membership downloads, you are logged in for sure.  Keep in mind that you will only be able to view content based on the membership level you are at.  If you have done all these things, and still feel like there is a problem accessing something, contact us at support from the link in the description above the faq’s.