Thanks Aaron, no-one can get in. Everyone who tries to access the CRM page gets the same message.
Scott, I had already restarted the CRM Server, thought that might clear it up but it made no difference.
Muhammad, all the users seem to be correct, present in the AD.
I have since opened up a support case with Microsoft and after much investigation it looks like our DNS server has somehow become configured incorrectly. We're still investigating to know for sure, but we're due to continue on with it on Tuesday (Monday is a holiday here) and I'll post an update when I know for sure that it's the problem.
Thanks for the suggestions,
James