Hi Manish,
Just curious to know if you are able to login to the switch successfully through CLI or was it stuck in pending-aaa as Bill pointed out.
if you are able to login to the switch successfully, does the CLI prompt still shows that it is in stack?
if so, have you tried "disable stacking" and reboot?
I understand unconfigure switch all should have wiped out the stacking configuration. still, it might be worth a try to see if you can disable stacking and reboot.
upgrading from bootrom should help clear all the contents in the switch. Let us know how it goes!