First try factory reset on settings screen that you navigate to from home screen. You can also do factory reset by removing outer back so you can see paper clip reset hole. With reader turned off hold paper clip in on pin hole until you can feel reset switch operate and while still holding in paper clip turn on the reader. This should return reader to original from factory condition.
If this doesn't cure your Mini's problem you need firmware image from working Kobo Mini or request a copy on this forum.
Download these 2 programs.
http://www.vconsole.com/USB%20Duplic...%20Creator.exe
http://www.vconsole.com/USB%20Duplic...e%20Writer.exe
The Flash Drive Image Writer will write the firmware image to SD card. Some Kobo Mini have 2GB Micro SD card Some Kobo Mini have 4GB SD card with only 2GB formatted. You can write Kobo Mini image to larger SD card then expand partition using G-parted or other utility filling up to 32GB SD card.
Flash Drive Image Creator will copy a good Kobo Mini SD card image to your computer so you can later recover from corrupted firmware image. This is the ultimate firmware backup if your trying other than Kobo firmware on your Mini.
https://www.mobileread.com/forums/sho...d.php?t=200866