Drupal Community Portal

Hi all,
I'm having a horrible problem where when editing a page, after clicking "save" it brings up a page (the theme still shows, but no menus) that says "Page not found". This get's annoying after a while...
I'm not sure what changed as it was all working fine and funnily enough, still works on some pages sometimes. But generally it gives me that error. I really need to finish this website and I can't edit anything, so it's a really big problem.
Please help if you can.