Please, after upgrading a cisco 3660 Router, and reloaded, the Router unit is not coming up again. Can any one kindly help with how to bring up the router?
You should start the router in rommon (whit key escape sequence)and put the original IOS.
If the router is working correctly after reload with original IOS, then the IOS that you had loaded for upgrade was not accurate.
See Cisco for rommon procedure.