ESX 2.5.3 hanged while booting up at "VM Kernel"

[sunhux G]

Our VMWare server ESX 2.5.3 self-crashed & failed to boot up.
It stopped at the point "VM Kernel ..."  while booting up

Is this a hardware or ESX OS fault?

If OS fault, is there any way to copy over a good kernel or
boot up from single user to recover from it?

We don't have any backup of the ESX, only the Wintel clients
(ie the VMs are the only ones with backup)

I was told in the past we just need to reinstall ESX but now
we can't even find the old ESX 2.5.3 media (just shifted office/
datacentre).  Can we use the ESX 3.5.3 media (but was told
we don't have the licence)