Is the trunk port connected and configured with the correct speed and duplex settings?
Issue the show port [module_#]/[port_#] command, where module_# and port_# are the module and port numbers for the trunk port in question. Check the output in the Status, Duplex, and Speed columns. The status must be connected and the speed and duplex must match on both ends of the trunk, or one or both ends of the trunk can be set to auto-negotiate the speed and/or duplex.