To manually fix Kindle E999 error: keep holding the power button down until the screen goes black, and then you can release it. This should reset your Kindle, and when the Kindle restarts, make sure it's charged up and connected to a stable Wi-Fi connection. If that doesn't work, you can try unregistering your Kindle from the settings option and then re-registering it after you fixed the E999 error.

Manual solutions have caveats. If you have a firmware corruption issue or hardware issue inside the Kindle, they may not solve your problem and you should probably talk to a technician or do a factory reset in that case.

Remember to backup your data on your Kindle as well. MacSonik EPUB Converter can help you quickly transform EPUB to PDF, DOC, JPEG and 10+ different formats in bulk if you want to keep your data safe. The MacSonik EPUB Converter allows you to backup the data on your Kindle without compromising on data integrity, folder hierarchy, and the ability to do it all in bulk.

Steps to Backup Your Kindle Data (EPUB):

1. Open the MacSonik EPUB Converter Tool.
2. Add your EPUB files to the interface.
3. Specify PDF as the output format.
4. Select the export folder to save the ebook files to.
5. Click Convert.