I've been using Kindle for years, and sometimes books just refuse to open for no apparent reason. The most common issue is file format compatibility. Kindle supports formats like MOBI, AZW, and PDF, but if you sideloaded a book in EPUB or another unsupported format, it won’t open. Check the file extension and convert it if needed.
Another possibility is corrupted files. If the download was interrupted or the file got damaged during transfer, Kindle won’t recognize it properly. Try re-downloading or transferring the book again. Also, ensure your Kindle software is up to date, as outdated firmware can cause compatibility issues. If none of these work, restarting the device might help—sometimes it’s just a temporary glitch.