Perhaps it's not responding. Kill the program/process and try launching it again and afterwards, don't do anything and let the client load. You can also try closing your other programs such as your browser, Skype, etc before launching the client.
Mine also launches windowed, and says Not Responding, then a second or two later it starts showing the intro. Have you checked the shortcut to see if it has '-w' or '-window' at the end?
The client is supposed to start windowed now. It seems like you guys are confused about that, but I might be wrong. Client improvements: The game client will now launch in windowed mode by default. Fixed issues that caused the client to give a 'Parameter is incorrect' error when launching the game client. https://royals.ms/forum/threads/new-source-update-19-19-04-17.89932/
