So how are you factory resetting? Boot into recovery mode, bring up the recovery menu, select factory reset, right? If you are doing anything else then tell us what, but that's the only way of doing an actual reset if you can't get in (assuming you are using a stock recovery, but someone with a custom recovery would probably have said so in these circumstances).
A bit more background, like which HTC One model and what you did prior to it getting in this state, might help, but confirmation that you are doing the above, so that we know exactly where we stand, would be the first step.
