Shut it off completely by pushing the power button down for at least 20 seconds. Now plug in ur fastboot cable into the fire hd and connect it your computer. Does it get into fastboot mode? Do u remember how much your battery was charged before it stopped being detected on your computer? If it was less, you would have to look for a way to charge your battery first, as I don't think the device charges itself in fastboot mode. If you remember having plenty of battery and you are sure you have a kindle fire HD 7 2012 edition then look for a boot loader repair guide which we have on this part of the forum of the XDA. If you are capable of performing the mod, good, else find someone to do it for you.