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If you have a laptop with a Fn key, it may suddenly happen that you're typing numbers instead of letters and you need to hold down Fn to type letters.
The solution to this problem is very simple, and it applies even to USB keyboards with Fn key.
Just press Fn + NumLock or Fn + Shift + Numlock. If you're using a Lenovo or IBM laptop, press Shift + Numlock. If that doesn't work, then your keyboard is broken and you need to get a new one.
This post was last updated on March 10th, 2013.