None of my single posts work I get error : URL not found

My blog is working after a period of several days being offline due to an internal server error. I fixed it (well, not “I” but a friend of mine) by deleting the .htacces file and then reinstalling.

Now it looks OK. My frontpage loads normally, but when I try to visit individual posts I get an error page that says:

Not Found

The requested URL /la-nueva-unidad-nacional-2/ was not found on this server.

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

Apache/2.2.9 (Unix) mod_ssl/2.2.9 OpenSSL/0.9.7a mod_auth_passthrough/2.1 mod_bwlimited/1.4 FrontPage/5.0.2.2635 Server at http://www.pabloalvestegui.com Port 80

What shoul I do?????