To be honest , you don't really give enought information to go on , however try these.
1.Have you checked the config on switchport hasn't changed , i.e the phone is not in correct vlan
2.Is this the only phone having an issue .i.e it could be faulty.
3.Have you ruled out the cabling? i.e do you know its not a cable fault.
4.Have you done any upgrades recently to callmanager of firmware? is it related?
5.Have you powered off the phone and back on to see if that works?
cheers
mark