The spacebar, “n” key, and “b” key, and “?” key on my friend’s computer keyboard are not working. What could be the problem?
Keyboard knackered. Sometimes they just don’t last very long.
just ask your friend to buy a new one, it is just NT$ 200- NT$300 max for a keyboard 
Go for mechanical keyboards, they last longer.
What is a “mechanical” keyboard ?
they are the once which make “click” sound when you press a key ( sound come from the keyboard 
).
these are better if you type a lot, they help to prevent CPT in your wrist.
ponchi
Think IBM PS/2-like keyboards. See surplus-sales.cc/ibmkb.html.
There’s a keyboard-dedicated store in KuangHwa market that sells these Lexmark keyboards, as well as other mechanical keybaords.
Beer is the keyboard’s gretest enemy. Many great keyboards I have had in the past had to be retired from active service because my beer refused to stay in its can or bottle.