Category (.html) page not available

[will3]

It seems that several html-pages cannot be displayed. In the admin panel I created a new category, and on the store homepage that category shows up. When I then click the category url, which ends in.html (so url http://www.mysite.com/category1.html), I get an error message saying:

"The page you requested is NOT AVAILABLE
A general error has occurred"

This also happens with other .html pages (like about us, policy, etc).

How can I correct this? Is this .htaccess related? If so, what do I need to do? Note I am not a technical expert or programmer.

[gjh42]

Stock Zen Cart URLs never include .html, so you must have a URL rewriter ("SEO") installed. This is apparently malfunctioning. Turn it off in admin and see if things work correctly.
Seeing your site live is the only way we can start to diagnose the problem if that does not fix it.