The firmware is more than just the 3.31.19086 package you downloaded. To completely replace the existing firmware, you will need to request an image for the N418 Libra 2 by posting your request in the
Kobo µSD Card Image Requests thread.
Before obtaining the image, you may want to try using the instructions in the
Memory Upgrade Mod Guide (With Pictures) thread to create an image from your existing µSD card and then writing the image to a replacement µSD card to see if that will revive your Libra 2.
I do not recommend re-using the existing µSD card since all too often it is the source of the problems. If you do want to try re-using the old µSD card, using the SD Card Association's
SD Memory Card Formatter for Windows/Mac and do a full format on the card before writing the image to it.