Root Cause:
Check your WCF Service application pool it might stopped
Check FAST Server admin account changing pssword.
Open FAST Search PowerShell and run below command.
Get-FASTSearchContentCollection.
Current Logged in Account Should be part of FASTSearchAdministrators group.
Whne yu install FAST Search this Group will "FASTSearchAdministrators"create.
Determine whether the FastSearchAdminAppPool administrative service is running.
Log on to the FAST Search Server 2010 server as a FAST Search Server 2010 user.
Click Start, point to Administrative Tools, and then click Internet Information Services (IIS) Manager.
Expand FAST_Search_server_name, and then expand Application Pools.
Right-click FastSearchAdminAppPool, and then click Properties.
If the service is stopped, click Start.
Determine whether the FASTSearchAdminServices site exists:
Log on to the FAST Search Server 2010 server as a FAST Search Server 2010 user
Click Start, point to Administrative Tools, and then click Internet Information Services (IIS) Manager.
Expand FAST_Search_server_name and then expand Sites.
Verify that the FASTSearchAdminServices site exists under Sites.
