“Another potential reason why a “hard” reset (or even yanking the power cord) will not recover a system has to do with non-volatile memory, like Flash. In this case, even removing any batteries won’t help, because the erroneous information can only be altered via a re-programming.
“That’s why re-installing software is often the only option. Even then, certain designs may have memory regions that are unrecoverable even through this technique. For example, some Flash devices have a boot sector that is ordinarily only programmed in the factory. At that point, the only solution may be to physically replace the memory.”