The "Function" button, or the Fn button, is a key on compact keyboards such as those of the laptop computers. The button acts as a modifier and is used much like the "Shift" key, for operating two or more functions in a designated key that would otherwise be kept separate on full-sized keyboards.
The "Function" button is in the lower left corner on the keyboard, either to the left or to the right of the "Control" key, with the red or blue "Fn" print on the button. You will want to know how to turn this button off when your keyboard is stuck in Function mode.
Clean thoroughly underneath and around the "Function" button with a can of air duster. Dust or other debris may be in contact with the key circuit when sandwiched between the button and the keyboard plate. This makes the laptop misinterpret that the "Function" button is depressed.
Locate the "Number Lock" key on your keyboard, often with the printed words "NumLk" or "Num Lock." The "Number Lock" key controls the functionality of the "Function" button, similar to the "Caps Lock" key that controls the letter keys.
Press the "Number Lock" key once to disable the "Function" button. You should see the "Number Lock" key light on your keyboard turn off. On some layouts, you will have to press the "Number Lock" key with the "Shift" key or press the "Function" button with F11.
Be sure nothing is pressing the "Function" button when you type and you do not accidentally press the "Number Lock" key.
The "Fn" button is sometimes found in full-sized keyboards as well, named "F-Lock."