You can create two subvi's that are configured to show front panel when called. One of them will be for a valid login, the other for an invalid login. In your main, put the login as the first state. If it is valid, make your next state the one that calls the valid vi. If the login is not valid, make your next state the one that calls the invalid vi. Then when the subvi's return, you can process whatever returns they have or just simply exit the state machine while loop.
If there is no data to process in main, if the two subvi's are totally separated, you can get by with a simple case statement instead of a state machine. In this architecture, you would run a login screen first. Then you validate the login, and have a boolean output. Wire the boolean to a case structure. If True, call the valid subvi, if False, call the other one. This is simpler than a state machine. Just be sure to configure the subvi's to show front panel when opened.
tbob
