Not Acceptable?

[tobehumanis]

I'm fairly new to all of this and I must say that so far everything has been working perfectly until I went back to edit some items I had previously set up in the catalog and received this message:

 

Not Acceptable

An appropriate representation of the requested resource /catalog/admin/categories.php could not be found on this server.

 

I didn't go anywhere near this file and it still exists. I'm lost and any help is most appreciated.

Thanks