Since April 6th, Google's index had been suffering from an unfortunate bug. Affected web sites had pages dropped from Google's index by the issue (the cause of which they have yet to elaborate on). Those sites affected had URLs stop appearing in search results for any keyword, costing their owners very real traffic and lost business opportunity. John Mueller, Google's Senior Webmaster Trends Analyst confirmed the issue on Twitter:

Sorry -- We had a technical issue on our side for a while there -- this should be resolved in the meantime, and the affected URLs reprocessed. It's good to see that the Inspect URL tool is also useful for these kinds of cases though!

— ? John ? (@JohnMu) April 6, 2019

Google announced that they'd resolved it the following day, but that turned out to be premature. More websites were seeing their URLs removed from the index, proving the issue was not fully resolved.

Finally, late Wednesday night, Google's Search Liaison, Danny Sullivan, reported through his professional account that they'd fixed the issue:

 

The indexing issue has now been fully resolved. We apologize for the inconvenience. We appreciate your patience as we restored normal operation.

— Google SearchLiaison (@searchliaison) April 11, 2019

  What should you do?

First, check to see if you've been affected. While Googlebot will eventually recrawl your site as part of their normal operations, there's no reason not to get ahead of it. 

• Run a Google search for site:yoursite.com to get a results page of everything you've got in the index. You can narrow this search by adding important terms to the end, like site:yoursite.com "important keyword". If your pages are missing, you need to verify it and submit for reindexing.
• Head to your Google Search Console (if you don't have this free account set up, now's a great time.)
• Under 'Overview' on the left, you'll see a link for 'URL Inspection,' a new tool that was made available to everyone last July.
• Verify the status of each of your missing URLs. If they're not in the index, click 'Request Indexing'.

Pages should eventually return to their original ranking in results. That being said, remember that Google's algorithm doesn't aspire to get every page into the index - only those of useful value.

One thing to add here - we don't index all URLs on the web, so even once it's reprocessed here, it would be normal that not every URL on every site is indexed. Awesome sites with minimal duplication help us recognize the value of indexing more of your pages.

— ? John ? (@JohnMu) April 7, 2019

Also, remember that the core algorithm component, called Hummingbird, and the machine learning component, RankBrain, have been working in the meantime. Changes to a page's rankings for certain keywords don't necessarily mean that it's due to this bug.