Unable to type the number 0 on the keyboard | Causes and solutions

Tonight, while talking on the phone with my daughter for the first time in a while, she told my wife and me that she had run into a pretty serious problem on her computer. Even though it was a trouble story, she sounded strangely cheerful and excited as she said, "Hey, listen! When I tried to sign in to Windows on my laptop, I couldn't type just the number 0 on the keyboard!"

According to my daughter, she didn't know why, but when she tried to sign in to Windows 11 as usual, she kept getting a password error and couldn't log in. Her Windows login password is a simple numeric password, so she tried entering the numbers one by one slowly--and discovered that only the 0 key wouldn't input anything no matter how many times she pressed it.

Restarting the PC didn't fix it. She thought maybe waiting a bit would help, so she left it alone for a while, but nothing changed. She became so frustrated that her head was practically boiling. Eventually, she noticed that besides the row of number keys (1 through 0) at the top of the keyboard, there was also a 0 printed on the M key (the key that normally types the Japanese "も"). She thought, "Could this be it?" and pressed the 0 on the M key--sure enough, it worked, and she was able to log in to Windows.

My daughter had been completely stuck because she had always been able to type 0 using the top-row 0 key, and suddenly it stopped working. But she seemed genuinely happy that she managed to solve the problem on her own, and she excitedly told my wife and me the whole story over the phone.

While I found her story adorable, my wife and I almost simultaneously said something like, "It's strange that the 0 key stopped working. There must be a reason!"

My wife loves troubleshooting computer problems, so she became very interested in this issue of "only the 0 key not typing anything" and happily began investigating. She told our daughter, "Take a picture of your keyboard and send it to me right now," and our daughter sent a photo via LINE.

A photo of my daughter's Dynabook laptop keyboard, taken by my daughter.

Looking at the key layout, I saw that there really was a 0 printed on the M key. Until I saw this photo, I honestly suspected that my daughter had simply mistaken the number 0 for the letter O. Daughter, I'm sorry for doubting you. On both of my personal laptops, there is no 0 printed on the M key. Even my gaming keyboard doesn't have a 0 on the M key. I never imagined such a layout existed until I saw the Dynabook keyboard--probably because the keyboards I usually use are different from Dynabook's layout. (I used to be a Dynabook user myself. If I haven't thrown my old Dynabook away, maybe I'll dig it out someday and look at the keyboard layout again.)

Before my wife began her full investigation, I said, "Maybe NumLock is on?" But my wife replied, "Wouldn't NumLock turn off automatically when Windows restarts?" So we didn't check further at that moment. (It turns out this assumption was wrong--the cause really was NumLock. And we also learned that on my daughter's laptop, restarting Windows does NOT disable NumLock.)

We told our daughter, "We'll look into the cause, so wait for our call," and hung up for the moment.

After researching, my wife discovered that the Dynabook laptop my daughter uses does not have a dedicated NumLock key. However, pressing Fn + F11 performs the same function as toggling NumLock.

My wife called our daughter and said, "Try pressing Fn + F11. Can you type 0 now?" Our daughter shouted joyfully, "It worked!"

Here is the summary:

The reason why only the 0 key cannot be typed may be because NumLock is enabled. In this case, the solution to the problem of only the 0 key not working is to disable NumLock. On laptops like my daughter's Dynabook, which do not have a NumLock key, pressing Fn + F11 will disable NumLock instead. Depending on the PC model or settings, behavior may differ, but it seems that NumLock does NOT automatically turn off when Windows restarts. So if someone accidentally enables NumLock, they must manually disable it.