Sometimes a Cisco phone will loose its configuration, sometimes after a power cut, sometimes after the internet connections changes, sometimes for no reason at all!
If this happens, you can follow these instructions to factory reset the phone:
Fully reset the phone by holding down # (hash) while plugging in the phone.
When the lights flash in sequence down the right hand side, release # and type:
If you’re a customer of ours, your phone will be configured to download its configuration from our provisioning server.