Clear your calculator of the last equation you did by pressing the "Clear" button.
Click the "Mode" button and use the arrow keys to scroll over "Normal" mode. This will set the calculator up to do basic equations versus any long algebraic equations, or other types of equations on which your were previously working.
Practice setting the mode to normal when your are done using your calculator so you always know what mode the calculator will be on when you turn it on next time.