Alfresco can be quitte a challenge if you are new to it.
Some tasks that should be simple can be very hard to setup.
One of those issues can be handling a site where the owner (siteManager role) isn't in your company anymore or left the share site without appointing a new siteManager. In this case you are left with a site that nobody can handle.
To get ownership of the site follow the steps below.
I do this per alfresco documentation example.
1. Login alfresco explorer.
var site = siteService.getSite("SITENAMEHERE");
var authorityName = "YOURNAMEHERE";
var role = "SiteManager";
3. Alter above to reflect the sitename you want to alter and your username.
4. Goto a random document and click details.
5. choose run action.
7. finish it et voila you are a siteManager of the site you want to and can manage it from alfresco share.