If ESXi detect the LUN incorrectly then it do not allow to mount the VMFS volume without deleting exiting VMFS volume and formatting LUN.
In below Image we can see, Keep the existing signature and Assign a new signature Options are grayed out and only Format the disk option is available.
If we select Format the disk option, all the VMs will get deleted from VMFS volume.
We need to mount such VMFS volumes forcefully.
1. Login to ESXi host using local shell or using SSH with root username and password.
2. Find VMFS volume ID of affected LUN.
3. Then Mount VMFS volume with UUID.
#esxcfg-volume -M UUID
e.g. #esxcfg-volume –M 5135cc26-5a43cc6d-34c7-0025b5990306
This will forcefully mount existing VMFS volume on ESXi Host and then you can use this volume to run VMs.
See this KB for more information -