Why do I have to hold down the "Fn" key to get normal letter?

Here's a shot of my laptop's keyboard (please, ignore the dirt - it's a well used, and well traveled, machine):

and down in the lower left you'll see the "Fn" key that we're talking about (though mine's blue instead of purple):

If you look closely at the rest of the keyboard you'll see that several keys include blue lettering on them as well. In fact, if you look at the keys 7, 8, 9, 0 on the top row, U, I, O, P on the next, J, K, L, : on the next and M, > and ? on the bottom, you'll see that each has a blue character on it:

That layout of blue characters mimics the layout of the numeric keypad on a full-sized keyboard.

Normally, if you hold down the "Fn" key, you'll get whatever character or function is printed in blue on those keys. So holding down the "Fn" key, you now have a numeric keypad in the middle of your keyboard. Release "Fn" and things are back to normal.

Which is exactly the opposite of what the question is asking. The person asking needed to press the "Fn" keys to get the normal, white, characters for each key.

The "problem" is simply that there is a persistent toggle. If you press "Num Lk" (I've also seen it labeled "Pad Lk" for numeric Pad) it turns on the blue keys, and inverts the meaning of the "Fn" key - it's a way to use that embedded numeric keypad without having to hold down the "Fn" key the entire time.

To fix it, just find and press the "Num Lk" or "Pad Lk" key again. (Note, sometimes these keys are themselves "Fn" keys, so you may, or may not, need to also hold down the "Fn" key to get the right function.) Often there is an indicator light associated with the setting.