All my posts are 404 – please help?

I upgraded to 2.0.9 recently and my customisation disappeared – I know I need to find the files and make the changes but someone one else set this up for me and now I need to work it out myself.

In WP I can see all my posts but the previews are all 404s. I can edit the posts but the views area all 404s.

I’ve tried to search for an answer but I can’t find one.

Thanks,
Kath