I really need your help in resolving this, my ranking dropped, I diagnosed and it came that there were around 80 - 404 pages in google webmaster tool. They all exist.
If "fetch as Google" is able to retrieve the pages without errors, then it sounds like Googlebot just hasn't gotten around to re-crawling all the pages yet. The errors will go away within a couple weeks as Googlebot recrawls the error pages.
You can use the "mark as fixed" feature in Google Webmaster Tools. It will remove the errors from your list for now. If Googlebot ever has problems with the pages again when it crawls, it will show you new errors at that point.
Until Google crawls and indexes those particular pages again, they won't get any search traffic. So it will be hurting your SEO from that standpoint until Google finds them again.
Luckily 404 errors on your site don't hurt your other pages an your site. Google's John Mueller says:
404 errors on invalid URLs do not harm your site’s indexing or ranking in any way. It doesn’t matter if there are 100 or 10 million, they won’t harm your site’s ranking.