That can also happen if you have a corrupted ebook that hangs the importing process. OTOH, the import process starts at 6% and ends at 94% so it may not even be starting the process.
Since an eInk screen is bistable, the image will stay there until it is rewritten.
You might want to try the
Manual reset your Kobo Clara HD, Kobo Nia, Kobo Elipsa, Kobo Clara 2E, Kobo Elipsa 2E procedure from Kobo's website.
The usual warnings that this will wipe your ereader back to factory apply.