my routerboard is died after ubgrad

when i wana change the software from 5.19 to 4.17to router 750gl the router died and not open from winbox no ip no mac
what i can do
and how can make serial cable to conect the router to computer
if we can write flash can any body send me the soft for flash and i write it
the flash no samsung 001
k9f1208u0c
pc80

rb750gl
thanks

use netinstall please.

how i can use netinstal
and the router didnot have serial cable to conect the router to computer
plz can any one replay to me