Sounds like the phone was hard reset... If you issue the command 349167285*0# the firmware on the phone gets completely wiped and the phone will just sit at a blank screen and look like it is dead. It should eventually pull its firmware again but this process can take up to 15 minutes. If it doesn't you have to do some more to get it to work.
If the phone is on a really old version it will not upgrade to the latest software that exists on your UC500. In this case you will have to step upgrade it before it will come back from the dead. Phones that have pre-8.3(3) firmware installed must be upgraded to 8.5(2) firmware version as an interim step before they are successfully upgraded to a 9.X firmware version.
Easiest way to do this would be to download the 8.5(2) firmware files from Cisco.com, set up a local TFTP server on you machine and then change the DHCP pool option 150 on the UC500 to your computers IP address. This requires the CLI which is dangerous if you don't know what you are doing. If you know what you are doing issue the following:
*****
!
conf t
ip dhcp pool phone
option 150 ip XXX.XXX.XXX.XXX
!
*****
Reboot the phone and it should begin pulling the interm firmware and you will see TFTP requests in the debug. If you don't check your firewall or turn it off completely and make sure you only have one interface enabled.
Once the phone is back alive change back the option 150 value to the original ip (10.1.1.1).
Reboot the phone and it should pull its configuration and upgrade to the version on your UC. If it doesn't issue a soft reset (hold # at boot and when lights flash orange key in 123456789*0.#). This will kick it into the upgrade cycle and it will retrieve the 9.X firmware from the UC500 tftp.
This isn't a definitive guide and there may have been some steps left out. Bottom line is you need to load 8.5(2) via TFTP and then reconnect to the UC500.
