WordPress 404 errors – archives missing

Hello,

I have a blog which has wp 1.5.2 on it. i have been posting for sometime know and noticed all my historical posts are gone. the archives i guess.

i have a few categories. i tried making a new post, and when i click on the category button for this post, which i just made..it says

/category1/mytopic/ was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

so i guess, i am wondering what happend? how can it be fixed? and in which folder is the wp archives stored? i have it installed on the root domains

when i go to manage posts, i can see all the posts, categoires, and everything. its just i guess it posts but not updating

i have contacted my isp and they made no changes.

tia