unless "umount -l" solves your problem, you can set up a fake server with the same address as the one that has gone away - but you don't actually have to set up a new sever or anything. The easiest way out of the blocking/hung umount situation is to set up a local alias IP interface, as follows:
ifconfig eth0:nfstmp 188.8.131.52 netmask 255.255.255.255
umount -l /mnt/deadnfsmount # -l or -f or whichever that gets the job done
ifconfig eth0:nfstmp down
(obviously 184.108.40.206 being the (former) IP address of the (now dead) NFS server)
It might be wise to add the intr option to any /etc/fstab entries that might end up hanging or crashing. If you don't use the soft or intr options, then when the server hosting the NFS files goes down, the server on which the files are mounted (the client) may hang when booting up.
According to man 5 nfs:
soft / hard
Determines the recovery behavior of the NFS client after an NFS request times out. If neither option is specified (or if the hard option is specified), NFS requests are retried indefinitely. If the soft option is specified, then the NFS client fails an NFS request after retrans retransmissions have been sent, causing the NFS client to return an error to the calling application.
... and then it goes on to say intr is preferred over soft, but it has the similar effect of preventing hanging.
I've never managed to get umount -f to work. A useful trick is to set up another server mounting the same export, give it the same IP address as the old server. Your NFS client should think everything is back as normal and processes will unblock. You can then unmount the mount point normally and remove the IP address from the temporary NFS server.
For Solaris, restarting the NFS client will resolve the "hard mount spiral of death". The command for Solaris 10 is "svcadm restart network/nfs/client"
Haven't tried this on a Linux box lately (because those all mount with the "intr" flag so they rarely have this problem), but it probably will also fix the problem.
just an OS X-specific follow-up, since mount commands are mostly *nix agnostic: the -l (lazy) flag doesn't exist in OS X, however, the -f (force) flag does, and proved to be sufficient. Also, the system-generated mount points are in /Volumes (/Volumes/myserversexport)
I've met this same problem. Since the NFS server got removed, I can not umount the nfs from client.
I tried the following trick, see if it could be helpful.
Since the original NFS server is gone, I create a new server with same IP and exports. Then I try umount -f /mnt/nfs_part. I finally could umount the nfs now.