There has been a problem with 123 reg’s nameservers for most of today such that they aren’t responding to DNS queries.
This is pretty severe as it means anyone trying to access a site pointed at those nameservers will not be able to resolve the domain name to an IP address, and will be given a Server not found error. It will also affect any e-mails to those domains.
On their status page, they say:
17/11/2007, 09:15 – We are currently aware of a problem regarding 123-reg nameservers which in turn is affecting the visibility of websites and services such as email. At present, we have engineers looking into this as a matter of urgency. Please be assured we are working to rectify this issue as a priority and hope to resume normal service as soon as possible. Thank you for your patience in the mean time. We apologise for any inconvenience caused.
If you are looking for a new registrar, you could try our Livetodot domain registration service.