You can restart nfs using command “service nfs restart” in linux. The command could create problems sometimes. A better way to restart nfs and related service is :

Stopping NFS Services

service nfslock stop service nfs stop service portmap stop umount /proc/fs/nfsd

**Starting NFS Services **

service portmap start service nfs start service nfslock start mount -t nfsd /proc/fs/nfsd