If
1. You use WordPress;
2. You have permalink turned on;
3. You just wrote a page using WordPress but when you view the page, you get 404 page not found error;

Then
1. Go to Options->Permalinks;
2. Click Update Permalink Structure. If your .htaccess cannot be updated through the web interface (very likely), copy the text and replace what you currently have in that file. Say a silent prayer;) and hopefully it will work.

End If