Do you know if you have the Hot Desking option set?
Is Force Login clicked?
and a password set in Login Code?
Did the power go off, or the phone switch get rebooted?
This is what would happen if you're using Hot Desking/Force Login and you reboot the IP 500.
Or are you asking how to log in again, to get the name and extension number showing up again?
You may want to clarify your question.