Please try disconnecting the phone from the Ethernet port or router that it is plugged into and also disconnect the power cord on the phone.
Leave disconnected for 10 minutes and then re-connect the phone back to the Ethernet port/router and power. This should then provide the phone with an internet connection - meaning the phone will be back online ready to make and receive calls.
If this is not the case please provide us with your phones IP address so we can check to see the IP address has not become blocked by contacting our Support Helpdesk.
Please also provide us with your public IP address - just type into a browser 'what is my IP address' and send this to us so that we can check this is not blocked.
It may be that the phone has become unregistered from the SIP platform - in which case you may need to login to the Cisco web interface where you will need to check the line setup and if necessary re-register your VoIP line onto the phone as shown in the step by step instructions here.
In the event that we are unable to help you then please contact the Cisco UK helpline on 0800 4047778.