I'm no SEO pro ..the first thing I would look at are your robot text files and see if someone accidentally disallowed Google but not Bing and Yahoo. That would cause this issue and go unnoticed by a lot of people my SEO friends tell me. I would also look in Google Search Console and make sure Google can index and see my site ok.. make sure there are no errors as well. If that all looks ok, then maybe hire someone do an external audit as a fresh pair of eyes may be needed on the site.
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