All page and posts have ERROR 404 - Category pages with posts okay.

<big>Not Found, Error 404</big>
The page you are looking for no longer exists. Perhaps you can return back to the site's homepage and see if you can find what you are looking for. Or, you can try finding it with the information below.

This is only affecting POSTS & PAGES (eg. Services)

If you go to the categories (eg. environment) the posts are there. When you click the link to the stand alone post page it does the Error 404. Post excerpts are appearing on home page but disappear when the link is clicked

My dropdown menus from the tabs have also gone.

I had uploaded a few plugins at the one time (stupid me should have done one at a time and tested for compatibility). I disabled the latest plugins but nothing happened.

PLEASE HELP!! Should I delete the plugins totally or is disabling enough?

Thanks in advance - I have to get this fixed as I am applying for contracts!!