If Outlook has been working with the same username and password saved up (i.e you do have not needed to keep typing them in every time you checked your email)  till this problem occurred, it is highly likely that either your email service provider has had cause to change your password or they have problems with the email servers / service on their side.

I would suggest checking with the service provider.  You will often find that they have Webmail access.  This is basically a website where you can login and check your email.  If you service provider confirms that their are no issues with email try and have them reset your password.

The only other thing I can think would cause any type of problem of this nature would be if you had recently installed any type of firewall software.  If so try uninstalling this and try again.

Hope this helps