one day we lost access to ourSharePoint Central Administration. The reason of this was that someone deleted the BUILTIN\Administrators from the Farm Administrators group.
No problem, just add the BUILTIN\Administrators back to the Farm Administrators Group.
But how do you do this without access to the Central Administration?
Well, after some searching and googling I found a solution. You can use STSADM to add people to groups in a Site Collection and the Central Administration is just another Site Collection.
I know the suspense is killing you so here ‘s the STSADM command:
Lo mejor es tirar de TechNet, es lo más completo http://technet.microsoft.com/es-es/library/cc303422
Desde la Administración Central de SharePoint > Security> Manage the farm administrators group
New > Add Users