It looks like you redirected the default user login page to somwhere else.
When I go to http://www.gelpalhano.org/?q=user, I get a HTTP 302 moved temp, followed by a 301 moved perm, followed by a 200 OK to http://www.gelpalhano.org/site/ (what appears to be your homepage).
Generally clean-urls would fail because your .htaccess if not configured properly, or because your web server is not configured properly. I saw your web host's headers say that the server is of type Microsoft, so I would not be able to suggest a fix for your particular setup (I only use the LAMP stack).
I can suggest however that to regain back access to your site, you substitute
www.gelpalhano.org/?q=[the relative path to your user login page], and then once you've logged in, you can go to
www.gelpalhano.org/?q=[the path where you turn on/off clean urls] and turn them off while you trouble shoot the issue. IMHO, this is a server setup issue, and I think you have more chances of having your issue resolved by researching and posting the same question at serverfault.com. Good luck and hope that my answer helps.