Working on FreeNAS, Ive had to research a hung NFS mount everytime I reboot FreeNAS or something silly.
A trick to execute on the NFS client , is to add an alias to the interface with the IP of the NFS server.
In Linux the command for that is something roughly like:
ifconfig eth0:fakenfs 192.0.2.55 netmask 255.255.255.255
And then to remove it:
ifconfig em0 alias 192.0.2.55 netmask 255.255.255.255
ifconfig em0 delete 192.0.2.55