OWA Access Denied

From my office PC, I was able to access email using OWA successfully without any problem.

All of a sudden all users of our office are facing problems that they can not access emails using OWA. Users enter their user ids and passwords and when hit ENTER, nothgin seems to happen and Main screen asking for User id and password pops up again. After three unsuccessful tries, we get an error message: ACCESS DENIED.

Please note that under Outlook, we are still able to access our email without any problem.

All other users who are traveling other places are still able to access their emails on OWA so problem seems to be limited to our office. However, no parameter has been changed either in Server or PCs.

We are using DSL serive of an ISP and the proxy servers in Explorer are configured properly.

What can be reason of this glitch.

Regards
Tariq