Remote desktop and services that refrain from accessing it.

Just wonder how can a person accessing to a remote computer that is behind a firewall via Remote Desktop Connection and all function has been enabled? and the computer that used to be able to be access is no longer accessable after a windows updates..What are the services that is blocking the access? The information gathered from the web for enabling the access does not seem to works at all.. do anyone know how to do it??? Many thank!!!