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Google Search Console (Google Webmaster tool) helps you monitor your site’s presence in search results by understanding how Google views your store, Google Analytics helps you track and evaluate the productivity of your website activities.
If you've had it added earlier and found it hard to access, please refer to this post to check and remove that script and have it re-added: https://help.shopify.com/en/manual/reports-and-analytics/google-analytics/google-analytics-setup
Please follow the tutorials to connect with Google Analytics and Google Search Console when you have your theme upgraded: https://help.shopify.com/en/manual/reports-and-analytics/google-analytics/google-analytics-setup and https://sb.secomapp.com/docs/Google-webmaster-tools/index.html
Google Search Console and Analytic work by adding some script to your site to keep your site on watch. If you have switched your theme, the script Google adds to your old theme will not be valid anymore. Read more details here: https://www.antevenio.com/usa/what-is-google-analytics-and-how-does-it-work/
It happens possibly because Google doesn't know all pages on your site. And Google may have indexed your page under a different version of your domain name. For example, if you receive a message that http://example.com is not indexed, make sure that you've also added http://www.example.com to your account (or vice versa), and check the data for that site. If your site is new, it may not be in our index because we haven't had a chance to crawl and index it yet. If your site used to be indexed, but no longer is, you can read more information here: https://support.google.com/webmasters/answer/34441?hl=en