There are two machines: (a) is the NFS server and (b) is the NFS client.
When machine (a) reboots is there a way to recover the NFS client on machine (b) without a reboot?
I don't understand what you mean by the word
recover but if you mean to
remount then yes, it is as simple as
sudo mount -a.
You can try a
umount -f mountpoint
I have seen this a lot on my RHEL 7 machines connected to a Windows NFS server. The umount command will fail, but it will release whatever is keeping the client from operating.