If its showing 8-12, then that will be the only working times available. You can check this by clicking the "work weeks" tab just below where your screenshot ends. Then click "details" on the right hand side. If you then click on a day in the pop up menu, it will tell you what the working times are for that calendar. NOTE that this is set as "standard" unless you decide to change it.
It appears that somehow the afternoon shift has been deleted from the standard calendar. If you want to reset it as default, click in the day you want, then click "Use project default tomes", which will reset the selected day or days back to the normal 2 shifts.
Its also possible that if you have resources allocated, these could have their own calendars, but unlikley if you havent entered anything.