When I try to type a "j" the character appears as a "1", "k" is a "2", and so on. It happened after I plugged in an external 10-key keypad. After unplugging it and restarting the laptop, I still have this problem. How do I unlock this key or fix this problem?

May need to do FN and a number, different on different machines. There are other methods.

Number lock is on.

Check the FN key and press the FN key and try to change the setting of your keyboard.
It can happen because of some keys pressed continues and keep on the hold position.