In most cases this common message indicates that the system has requested a service from an NFS server that is either down or extremely slow to respond. In some cases this message indicates that the network link to this NFS server is broken, although usually that condition generates other error messages as well. In a few cases this message indicates NFS client setup problems.
Check the non-responding NFS server for the need for machine repair or rebooting. Encourage your user community to report such problems quickly but only once.
For more information, see the chapter on NFS troubleshooting in the System Administration Guide, Volume 3.