If an NFS mount is frozen because the endpoint is unavailable, you can try a "lazy umount" on the mount to let the kernel take care of cleaning it up, rather than letting user-space processes wait on it:

# umount -l /path/to/mount

Additionally, you can use the NFS "soft" option when mounting the share to prevent hard locking of this kind; see nfs(5). Thanks to Kevin Turner for this tip.