completely dead means stuck at boot or no sign of any powering on?
first thing I would try is a JIG, force it into download mode and see if u can flash the original firmware
for firmware download, I recommend sammobile.com. They also have guides showing how to do so If you are uncertain or new to this
If u dont own a JIG and not willing to wait for one to be shipped, u can build it your own. it is pretty simple, just couple of resistors and a micro usb port
oh, and dont be shy about what you have done. Provide the essential details always for better assistance