The app has an auto connect feature, ensure your wifi is on and that your connected to the same network as your PC running freestyler (For the techies it also has to be on the same subnet)
The app will first try and connect to the last know ip Address, if this is the first time or it cant connect it tries to connect to every address in your network.
When it does it saves the IpAddress for next time to quick connect.
Should you lose wifi, or Freestyler loses sync with the app then Freestyler will refuse to allow any connections to the server.
So if your unable to connect then check in the top right hand corner of Freestyler for a connection symbol, if there's one there then right click it and select disconnect All.
You will then be able to reconnect :)