Late last year, google+ announced that they will be shutting down due to lack of funds and activeness. They sent emails to different users using the google plus comment platform. At first, it was a pretty active platform with lots of groups created connecting millions of people. Google plus wasn't so active anymore, and decided to shut down which I think will help google focus more on updating blogger to have a better optimization and updated usage.
Why your comments are missing?
Google plus sent emails with download links giving everyone the opportunity to download your database from google plus and stating that If you've used Google+ for comments on your own or other sites, this feature will be removed from Blogger by February 4th and other sites by March 7th. All your Google+ comments on all sites will be deleted starting April 2, 2019. Learn more
READ THE FULL ANNOUNCEMENT: YOUR PERSONAL GOOGLE+ ACCOUNT IS GOING AWAY ON APRIL 2, 2019.
PS: I changed my comment system from google plus to blogger last year when I saw the message and all google plus comments are no more visible on my blog. You can not get back your comments. All you need to do is continue from here using the blogger comment system or any other commenting system you choose to use. It's so sad and hard but within months your website will look interactive again.
What to do
Simply change back to blogger commenting system if you haven't.
Note: that all your google plus comments will be lost and have already been deleted doesn't matter if you change or not, it doesn't exist anymore.
How to change back to blogger comment system:
Currently the option to choose between blogger commenting system and google plus comment system is no longer available on your blog setting. Google has automatically updated every blogger user platform to blogger comment system. By April 2 2019, every detail about google+ will be removed. It is a process so if your comments are disappearing at the moment google plus is still clearing off the database and it will be completed on the 2nd of april and google plus will no more be in existence anymore.
Go to your blogger settings
You will notice that the options to choose between commenting the systems is no more available. So kindly change and remove all google plus widgets.
This is an announcement google blogger share
Following the announcement of Google+ API deprecation scheduled for March 2019, Blogger will revert to only having Blogger profiles. Please change your profile from a Google+ profile to a Blogger profile here at your earliest convenience. Learn more.
How to change back to the blogger profile.
Go to settings, then click user settings.
When you click on save user settings, it will redirect you to this page.
After clicking switch to blogger profile, you will be redirected to this page.
Insert your username and continue to blogger.
Comment moderation: If you noticed, blogger has updated our comment pattern. You can now moderate your comments, delete and spam. It also shows your total number of comments excluding your google plus comments.
Read More about commenting systems.
Please share this post to help others resolve this issue.