Stu are you in Works or Runtime? I am going to ASSuME you are in Works.
Open the project. Under the Edit Mode tab in the project box double click the System folder. then click Node. You should see the name ,data source, Device, channel, station or server (if ethernet you will see the IP address), Timeout, OPC server,.
Make sure you are using a communication method that you have on you PC. Example if you are using DH+ make sure you have a DH+ path to use on your PC. If you are using a OPC angain make sure you are using a good path. If you are using RSLinx as an OPC Server make sure that path is valid.
if you can post copyof the project I might be able to help you with the comm. setup.