(Ensure vmkping x.x.x.x) - where x.x.x.x is your NAS IP - is pinging
Another workaround - below
NFS is a great storage solution within a VMware vSphere environment. But when the connection to the NFS server is lost, you’re in big trouble. In most circumstances some virtual machines are still connected to the NFS Datastore and since it’s in use, you cannot delete and recreate it. I’ve tried this by using the vSphere Client and trough the command line but didn’t succeed. In the end there’s no other alternative than rebooting the ESX host, or is there… If you want to find out how to remount an NFS Datastore you may watch below video. By Eric Sloof
Actual Trick is - NFS data store IP was x.x.x.x, he created a host name (using /etc/hosts file or DNS or alias host name on DNS) to fool the vCenter, then added host name when adding NFS source. vCenter thinks it's getting NFS from different Source..
May be it's a little dragging video, wait till end of video to see the insight