I have switched from one hosting company to another, maintaining the same domain (e.g. www.mydomain.com). Two days after DNS transfer was completed successfully, there was a sudden 60-70% drop in Google organic search results.
My site has some 20-30 static content ASPX pages and some 1100 dynamic content ASPX pages. (all can be accessed either as HTTP or HTTPS if the user has already logged in).
I have browsed through 100 result pages and found that Google has indexed also
mail.mydomain.com as well as https://mail.mydomain.com
I suspect the problem being too many (x25!) pages indexed. But How can I determine the REAL problem ? Is it a "BAD" IP assigned to me by the new hosting company ? or duplicate content ? or DNS configuration issues?
My feeling is it could be related to the DNS migration - i've seen this in a variety of instances. Usually the drop is short lived - things should recover in a few days. At least that's my experience...
Its always advisable to keep the old server up for a while after migration, maybe you took the old server down too soon and google was still visiting the old ip address. I think its something like that and usually will sort itself out but it won't hurt to put a robots.txt file on your secure server disallowing all bots. This can be done through .htaccess on Apache not sure how on a windows server.