I have recently rebuild ESXI 5.1 with update 3 but after adding the ESXI in to cluster, datastore did not comp up after the rescan option. i have one virtual machine hosted on that datastore, i just wanted to confirm if i add datastore once again. Will that process format my datastore and loose VM hosted on that datastore?
What datastore are you talking about? A local datastore or a datastore on shared storage? In case of a storage LUN, make sure that the LUN is properly presented to the ESXi host (e.g. using the correct/new iSCSI IQN).