Figured it out.
If you take the Star Tool and go all the way down to only 3 points for the star (first example below) and press Ctrl+Up Arrow Key, you will get that design. Never knew this existed.
To get out of this mode, you need to start drawing a star and press Alt+Up or Down Arrow Key.
I honestly don't know what you did back there to get those lines inside your star but I know a workaround till someone else comes with a more permanent fix for your problem.
- Select the star tool
- Click on the screen and add 5 to points section
- If you see the lines inside the star, select the whole star and go to Pathfinder > Unite
- Change the background color to yellow and resize the star to fit your needs