Keyboard keys not working? Find the cause, then fix it

First find out which keys fail, and whether they fail on another computer too. One dead key is usually dirt under it or a worn switch. A group of keys that stop together points to the keyboard's circuit or a spill. If every key is silent or slow, check a setting such as Filter Keys and the connection first. Keys that type the wrong character are a layout setting, not a fault.

Keyboard test

A keyboard reads its keys through a grid of wires, with each key where a row crosses a column. When one wire or its joint breaks, every key on it stops at once. On the keycaps that group can be a row, a column or keys scattered across the board, because the wiring decides it, not the printed layout.

1. Find out which keys fail

Open the keyboard test and press every key once. Keys that never light up are the ones the computer does not receive.
Note the pattern: one key, a few neighbours, a whole row, keys scattered across the board, or every key.
Try the keyboard on another computer, or another keyboard on this one. If the same keys fail everywhere, the keyboard is at fault; if they work elsewhere, look at this computer's settings and software.
On a laptop, plug in a USB keyboard. If it works, the built-in keyboard is the problem.

2. Settings that make keys seem dead

Filter Keys (Windows). Holding the right Shift key for eight seconds turns it on, and Windows then ignores brief presses, so keys seem to stop working. Turn it off in Settings, Accessibility, Keyboard (Windows 10: Ease of Access, Keyboard), and turn off its keyboard shortcut there if games keep setting it off.
Sticky Keys. Pressing Shift five times turns it on, and Shift, Ctrl and Alt then stay held after you let go. Turn it off in the same place.
Num Lock. With Num Lock off, the number pad moves the cursor instead of typing digits. On some laptops, Num Lock turns a block of letter keys into a number pad.
Fn Lock. On laptops and compact keyboards, it swaps the top row between F1 to F12 and the media keys.
Windows key lock. Many gaming keyboards have a key or an Fn shortcut that turns off the Windows key so it cannot pull you out of a game. A game mode in the maker's software can turn off other keys too.
Slow Keys and Mouse Keys (Mac). Slow Keys makes each key wait before it counts; Mouse Keys turns some letter and number keys into pointer controls. Find them in System Settings, Accessibility, under Keyboard and Pointer Control.

Keys type the wrong characters

If Z and Y are swapped, letters come out as symbols, or accents appear where they should not, the keys work: the computer is using another keyboard layout. On Windows, the Windows key plus Space and left Alt plus Shift switch between installed layouts and languages, and both are easy to press by accident. Remove the layouts you do not use in Settings, Time and language, Language and region. On a Mac, check the input menu in the menu bar.

3. The connection and the driver

Plug the keyboard straight into another USB port on the computer, not through a hub or the monitor.
For a wireless keyboard, charge or replace the batteries and move the receiver closer, away from USB 3 ports, whose noise can disturb 2.4 GHz receivers.
For Bluetooth, remove the keyboard from the computer's Bluetooth list and pair it again.
On Windows, open Device Manager, expand Keyboards, right-click your keyboard, choose Uninstall device and restart: Windows installs the driver again.
Close or update the keyboard maker's software. A profile or macro there can remap or turn off keys.

4. Dirt and stuck keys

Unplug the keyboard or shut the laptop down first.
Turn it upside down and tap it gently, then blow compressed air between the keys at an angle.
On a mechanical keyboard, lift the keycap with a keycap puller, clean around the switch and press the switch a few dozen times.
For a key that sticks after a sugary spill, clean around it with a cotton swab lightly dampened with isopropyl alcohol, and let it dry fully.
Laptop keycaps sit on fragile plastic clips: do not pry them off unless the maker's guide shows how.

After a spill

Unplug the keyboard or shut the laptop down at once, turn it upside down to drain, and let it dry completely before you switch it on again. Plain water often dries out without harm. Sugary drinks leave a sticky film that stops keys from registering and needs cleaning or a repair.

5. When it is the hardware

If the same keys still fail on every computer after cleaning, a switch, the circuit or the cable has failed. On a hot-swap mechanical keyboard you can pull out a dead switch and push in a new one; on other keyboards the switch has to be desoldered. A group of keys failing together usually means the circuit board or its ribbon cable, which is a repair or a new keyboard. Check the warranty first. On a laptop, a USB keyboard keeps you working until the repair.

When it is not a dead key

A key that types twice is chatter: see why a keyboard types double letters. Keys that fail only while you hold several others are a rollover limit: see keyboard ghosting explained.

Sources: Wikipedia, FilterKeys; Logitech, wireless products and USB 3.0 devices.