I am not sure I understand your question or how to help you with the information you provided...
The phones work but nothing else does? Are they digital or IP phones, if digital that would make perfect sense, they don't care what the IP address of the phone system is. Is the 5000 onsite?
Did the internal LAN IP scheme change? New router, different subnets/VLANs, etc?
Sorry, just need a lot more info here, remember that the Mitel in these instances is nothing more than another node on the network, if something changed, then the Mitel will need to get reconfigured.