We have one domain and one sub-domain. Until last week both of them appear in first page of Google search for very important keyword. Unfortunately Google dropped our main domain from search results. our main site has been in first page for 5 years!

About one year ago we build this sub-domain. It simply has been redirected to one of pages of main domain.

For solving problem we upload a independent site for sub-domain because we guessed that Google think this is our main page of our site. But problem did not solved.

What should we do? our main site offer main services and we we want that will be on first page.

Shout down sub-domain? Redirect to main site? Put the link of our main site in sub-domain? (About one year ago we put link of this sub-domain to our main site. Google indexed it and continuously bring that to top.) changing in robots.txt ....