Don't use Add Storage from the UI — it will wipe the partition table.
Resignature is non-destructive and creates a new datastore UUID but preserves contents.
Try, maybe like this, if possible:
SSH into the host if not already:
ssh root@<esxi-host-ip>
List all detected VMFS volumes:
esxcli storage vmfs snapshot list
Try resignature (safe, non-destructive):
esxcli storage vmfs snapshot resignature -l "<datastore-name>"
Or if the name isn't clear:
esxcli storage vmfs snapshot resignature -u <UUID>
try mounting it manually:
vmkfstools -V # rescan for VMFS
esxcfg-volume -l # list volumes
esxcfg-volume -M <volume-label or UUID> # persistent mount
Or try force mount (non-persistent, test only):
esxcfg-volume -m <volume-label or UUID>