How many vm as with this message as inaccessible? if only one vm then check in which datastore it is, browse datastore and go that location to see if your able to see all the files.
if all the files are located, right click on the vm, remove from inventory and add to inventory, power on the vm.
If multiple vms are inaccessible see, if all the vms belong a one single datastore, check the status of the datastore, if the datastore is inaccessible, perform a rescan and check vmkernel log in esxi, post it here.
refer the below kb for trouble shooting inaccessible VMs;