Typically a Windows Service runs in a non-interactive session (session 0). And, unless specific steps are taken a process started by a Windows service will also run in session 0. Since session 0 is non-interactive no process started in it can display a user interface. So although a Windows Service could start notepad in session 0, notepad's user interface would not be displayed.
Starting a process from a Windows Service in a logged-on user's interactive session so that a user interface is displayed requires the service to start the process with a token that contains the correct session id and also will have the necessary access permissions for the interactive session's window station/desktop. Accomplishing these objectives is not a trivial undertaking.
Its not clear to me from your description of the problem what you are doing. And, since this seems to be a question about coding I suggest you also tag the question for the programming language that you are using.