When creating a new session, you'll have an I/O Settings choice. If you set it to "Last Used" it will create your I/O's based on the last session you had open. If, for instance, you had a demo song, or a song created on a different type of system, the I/O settings will be incomplete or wrong for your particular interface.

To reset the I/O, first go to Setup menu>I/O Setup.
Choose he Input or Output Tab.
Hold Shift and select all your I/O labels on the left side of the screen.
Choose Delete Path on the screen. You'll likely be prompted that some of the I/O's are currently in use. It's fine to delete them, because you'll re-choose them.
Then click the Default button. This will create all your available default paths for the particular interface and samplerate you're using.
Repeat for the other Input or Output Tab if necessary.

With the I/O reset, you can now go to the Mix window an re-choose the correct input or outputs.