Hello my name is ***** ***** certified computer and web professional with over 10 years experience. I will be happy to help you.
The issue could be one of any number of things. The best thing to do to find out exactly what the issue is is to look at google webmaster tools here https://www.google.com/webmasters/tools/. Usually you can find the cause in there.
I am wondering if perhaps your URLs changes when you moved servers. For example if on your old website your URLs were something like www.website.com/products/new-products and now they are www.website.com/new-products that could cause this.
Basically check webmaster tools and see if there were any 404 page not found errors.
Another thing that comes to mind is if you either had too much downtime between server moves or you had the site running on two servers at the same time. Google can and will lower your rankings if you have the same exact website online in two locations.
Google actually has a guide for moving web hosts, it may be worth your time to look it over and make sure that all of these rules were followed and that could tip you off to what went wrong. As I said initially it could be many things and it is impossible for me to say for sure without having access to your webmaster tools. Have a look here and go through the guide and make sure everything was done as they suggest https://support.google.com/webmasters/topic/6033102?ref_topic=6029673&vid=0-635773995309932390-1704973830
You should be able to get your rank back in time however you must make sure that your site on the previous server is gone and also you are going to want to alert google of your site and ask it to re-index everything, this can be done through webmaster tools, here are instructions https://support.google.com/webmasters/answer/6065812?hl=en
Sorry I cannot give you exact reason but in this case there are too many reasons this may have happened so you will need to do some investigation. If you have someone who does your website for you pass along my information to them.
Overtime though your site should get the rankings back as long as you are not running it still on the old server or doing something else that is penalizing the rankings.
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