How do you get rid of a page that has hijacked your domain?

woke up this morning to see that the website of the organisation I manage was taken over by a new page?!!! how come?? None of the pages on our domain work any longer...Our domain name has not expired, even.

Check the DNS settings. Is the domain name still pointing to the correct server? If not, then change it back and then change the password to your DNS settings. If so, then someone hacked your web server and you need to find out how they did it so they don't do it again.