You may have programmed the IP phone to a static IP address that is already being used by another device on the network (IP address conflict) or the IP phone is set to a static IP address that is on a different subnet than the programming PC.  Try setting the unit back to default.  To do this, unplug the Ethernet cable from the unit.  Press and hold the unit’s “call” switch.  Plug the Ethernet cable back into the unit while keeping the “call” switch held in.  After about 12 seconds, four beeps will be heard.  Release the “call” switch.  About 5 seconds later, two more beeps will be heard.  The network settings are now set back to default, which includes enabling DHCP.