My site had 404 errors for 2 days because of some provider problems, which they fixed, but now my site isn't showing up in Google Search, but my results were very good for searching for a specific keyword...
What should I do for my site to regain its top Google position?
My site had 404 errors for 2 days because of some provider problems...
It's not normal for a provider to be messing around with your files on the server unless you requested them to do so. That's how I could see a 404 happening. I think the errors you mean are errors with HTTP status code in the 500's. Maybe 503. See: https://en.wikipedia.org/wiki/List_of_HTTP_status_codes for more info on the codes.
...but now my site isn't showing up in Google Search, but my results were very good for searching for a specific keyword. What should I do for my site to regain its top Google position?
As people stated, google does not like to index URLs that they believe are not easily accessible. (URLs that return any HTTP status code other than 200)
What you can do is go in Google webmaster tools, resubmit your sitemaps, then select the gear icon and site settings then select "Limit Google's maximum crawl rate" and move the slider to max so that Google can have a chance to scan your pages faster.
Assuming your pages were pulled because of a 404 error, then you should see the rankings return as Google re-cralws the page as others have suggested. If, as Mike suggested, it was another error (like a server outage) you might be in a different situation.
A few things to check:
1) First things first, do your pages return a status 200? If you use a tool like web-sniffer.net and enter in the different URLs, do you get a 200 status code returned? I'm guessing so given that you said the provider fixed the issue, but would be good to double check especially if you aren't regaining traffic.
2) In your web analytics program, have you lost traffic from Google? Or, are you just seeing the dip in rankings when you conduct searches or when you see Google related terms? Now that the issue is corrected, has that traffic changed at all (you might be seeing something different with rankings due to personalization)?
3) When you use the site:yourdomain.com in a Google search, as Simon suggested, do you see all the pages returned? If the pages are still there, click the green down arrow and check the "Cached" version. On the cached version, you can see the date it was cached - you want to know a) if it is the error that is cached or the actual page and b) if the date of the cache is after the provider changed something.
4) If your traffic is down and you are not finding the pages in the site: query, meaning things aren't coming back on their own, then the next step is to submit to Google via Google Search Console (google.com/webmasters).In the left sidebar click on Crawl then Fetch As Google. On the Fetch as Google tool, enter in a URL for one of the recently fixed pages. Click Fetch and see if Google is finding the right page. If it is, then you can Submit To Index. Here is more about the Fetch as Google tool: https://support.google.com/webmasters/answer/6066468?hl=en
5) Also in Search Console, go to Crawl then click Crawl Errors. Click on the "Not Found" tab. Are all of your errors listed there (or some of them)? Are all of the errors fixed? If so, mark the ones that are fixed as fixed. I've had limited success with this forcing a re-crawl of the error.
6) Last thing to check that may help. Are the URLs that were previously broken listed in you XML sitemap? If so, and assuming the pages are working correctly now, you can re-submit your XML sitemap in Google Search Console. This can (sometimes) prompt a re-crawl of pages. In Google Search Console, go to Crawl then Sitemaps.
Your site will return to the Google index once you fix the errors on your site and give Google a chance to re-crawl it.
In my experience it takes about twice as long to re-gain rankings compared to how long the error persisted. If there pages were 404 for 2 days, it might take a week for Google to stabilize previous rankings. If pages are 404 for a month, it might take a month or two after the error is fixed to get back old positions in the search results.
site:yourdomain.comas the search, only the rankings have lowered and its temporary.