Hi,
As per your scenario, as i said, i came across same situation where mine storage went down suddenly due to power outage.
Now the only option you have to take the storage down gracefully once and power on again. I am 100% sure, once storage will come up, your esxi will start reflecting the disappeared lun.
It is known with HP MSA storage as far as I observed. and if you go with storage vendor, they gonna suggest you to upgrade firmware first.
Take the storage down gracefully in off business hours.