This happens when the NFS storage provider gets unregistered from the vCenter server
Resolution
Register Netapp NFS storage Storage Provider (OTV) to vCenter:
Navigate to: https://<OTV IP>:8143/Register.html
If a security certificate warning appears, choose the option to ignore it or to continue to the web site
In the Plugin service information section, select the IP address that the vCenter Server uses to access VSC
Type the host name or IP address of the vCenter Server and the administrative credentials for the vCenter Server
Click Register to complete the registration.
Close the registration page after you complete the registration process because the web page is not automatically refreshed
Navigate to ONTAP Tools > Storage Systems. Confirm that the storage array in question is discovered and has a status of Normal. If not, add the storage system and allow discovery to complete.