This happened to me when an epub took a looooooong time to process, so that the battery went flat before the task completed.
The "fix" was to connect to the kobo to the PC; delete some epub files using the file manager and disconnect. After a few goes i found the problem file. I had a look at the file in sigil and found it had a huge .css with many thousand almost identical styles.
Similar problems have been reported in
this thread.