Clearly laid out here. Nothing nefarious. Just conspiracy theories from the right
https://www.fbi.gov/news/press-releases/statement-by-fbi-director-james-b-comey-on-the-investigation-of-secretary-hillary-clinton2019s-use-of-a-personal-e-mail-system
With respect to the thousands of e-mails we found that were not among those produced to State, agencies have concluded that three of those were classified at the time they were sent or received, one at the Secret level and two at the Confidential level. There were no additional Top Secret e-mails found. Finally, none of those we found have since been “up-classified.”
I should add here that we found no evidence that any of the additional work-related e-mails were intentionally deleted in an effort to conceal them. Our assessment is that, like many e-mail users, Secretary Clinton periodically deleted e-mails or e-mails were purged from the system when devices were changed.