You probably don't have permission to save the file in the selected folder ...which could be a permissions error if you think that you should be able to save there. Run Disk Utility and do a Verify of the disk, then a Repair Permissions ... unless you are saving to a network volume...
I have seen this when saving over a network and either the network has been disconnected and reconnected or changed in some way. I assume it has to do with the path name changing or some darker recesses of networking that I prefer not to know about. If this is your problem try doing a "Save as..." locally and then copy the file over the network.
