Have you ever made a few changes on your site, simple changes I mean, things like changing a URL or the site hierarchy by adding a category? If you have, perhaps you have encountered a loss of organic traffic to your website shortly after.
This happened recently to me on a website I operate as a pet project. The loss of traffic was not critical since it's just a site I test various SEO experiments with. In a nutshell, I decided I wanted to change my site links to a better overall structure. This is something I should have done with the site initially, but the site is over six years old, so I may not have been following best practices when creating it.
Fully knowing what could happen, I made the change regardless. I added some categories and changed my wordpress post type to show a custom structure:
As a result, many of the posts I had created in the past had their URLs changed. I knew by doing this it would create a bunch of 404 errors. To combat this, I could have added some redirects to those pages, but instead I wanted to see how quickly my traffic would take a hit. I made the URL structure change on Nov 5th, and as early as Nov 8th I started to see a massive drop in my organic traffic.
On November 9th, I changed the URL structure back to the original state they were in. I then logged into Search Console and went to the section Crawl Errors under the Crawl drop down. On this page, you will see any existing errors your site might be generating. You can check 404 errors (soft & hard) as well as any other errors (usually 405).
It should list all the pages that are generating errors. In my case, since I changed the structure back to the way it used to be, I was confident I could mark off all the pages as being fixed. To do this, select the pages which need to be fixed and click the big Mark as Fixed button. It is interesting to note that the consensus thought (according to some forums I frequent) about this button is that it does not actually do anything other than remove the dead link from your list in Search Console. I suspect there is a little bit more value to it than just a cosmetic list cropper, but with all things Google, we may never know.
After waiting a few days, I noticed a climb back to the previous rankings I originally had. Before I had tanked my organic search rankings, I wasn't really sure how long it would take to fix. I was actually expecting something closer to 2-3 weeks, but to my surprise it was a matter of only a few days before my rankings were corrected.