The basic syntax for this command is: appcmd start site "Site Name"
But you can start a site by id, by piping the output of 'appcmd list site' into the input of 'appcmd start site' using /xml on the former and /in on the latter.
I.e. appcmd list site -id:N /xml | appcmd start site /in
Open a command prompt using Run as Administrator.
Start site by Id:
C:\Windows\System32\inetsrv>appcmd list site -id:1 /xml | appcmd start site /in "Default Web Site" successfully started.
Stop site by Id:
C:\Windows\System32\inetsrv>appcmd list site -id:1 /xml | appcmd stop site /in "Default Web Site" successfully stopped.
About the /in parameter: