Thisis what a whois tells me:
Registrar:
eNom, Inc. [Tag = ENOM]
URL: http://www.enom.com
Relevant dates:
Registered on: 29-Nov-2009
Renewal date: 29-Nov-2011
Last updated: 30-Nov-2011
Registration status:
Renewal required.
Name servers:
dns1.name-services.com
dns2.name-services.com
dns3.name-services.com
dns4.name-services.com
dns5.name-services.com
So, to me it looks like you haven't paid your bill for your domain name. According to the above info you registered the domain with ENOM.com so they would be my first port of call.
Other than that check your emails and in particular check your spam folder for anything to do with renewing your domain registration - typically .co.uk are registered for up to 2yrs at a time.