This is generally a permissions problem. Perhaps a directory or file permission was changed while the client held the file open. Perhaps the filesystem's share or netgroup permissions changed. If the server were down or the network saturated, the "NFS server not responding" message would appear instead.
Log in to the NFS server and check the permissions of directories leading to the file. Make certain that the filesystem is shared with (exported to) the client experiencing an NFS read failure.
For more information, see the chapter on NFS troubleshooting in the NFS Administration Guide.