As with any computer software, if for any reason the software shut off suddenly due to a power shutdown (or any similar reason) while saving a file, the written file would be incomplete and become corrupt, refusing to open later on. It is strongly recommended to continuously make dated backups of your project file to be safe. If for any reason a backup was not made and you ended up with a corrupt file, you can simply follow the steps mentioned here.
A KPUB2 file is basically a ZIP archive. The first step to take is to verify that the file is actually corrupt. Rename the .kpub2 extension to .zip and open the archive using any zip file extractor of your choice (e.g. Winzip, Winrar, or your operating system's basic file explore). It should alert you that the archive is corrupt.
Zip recovery programs
Although the file is corrupted, it does not necessarily mean that it is missing data. All the data most likely is there, but only a few bytes written incorrectly could be causing this corruption. What you need to do is rely on professional ZIP recovery programs that recover as much data as possible from the archive. Every ZIP recovery program behaves differently (with varying tolerance to corrupt bytes) possibly giving you a different set of files.
We are impressed particularly with PowerArchiver 2016, which had recovered several corrupt project files for us in the past. It is not free but gives you a free trial days of 10 days.
Simply go to Tools > Repair Zip Archive, and select the .zip file (which was originally of extension kpub2). A fixed archive will be created in the same root as your broken archive (the filename ending with _PAfixed.zip).
Before renaming the extension to kpub2, first extract the files using your standard zip extractor program to check the files inside. Sometimes a file or two would be missing. If these are standard files, you can simply copy and paste them from another project (e.g. container.xml file). Some files are tightly related to the project (e.g. package.opf) but they can be constructed manually using any text editor.
After everything has been checked, select all the files inside the extracted folder, and compress it back into a ZIP archive. Rename the extension from .zip to .kpub2, and open it with Kotobee Author.
There are other free alternatives to PowerArchiver 2016 mentioned here: 6 Ways to Repair and Extract Invalid ZIP Files for Free.