First, check whether they reach the browser
Press all four arrows in the tester above. If they light up, the keys and the keyboard are fine and the problem is in whatever application was ignoring them.
If they do nothing here, the keys are not reaching the browser at all and the causes below apply.
Scroll Lock, which is only ever a problem in spreadsheets
In Excel and several other spreadsheet applications, Scroll Lock changes what the arrow keys do: instead of moving the selected cell, they scroll the whole sheet while the selection stays put. It looks exactly like arrow keys that stopped working.
Press Scroll Lock once and try again. On a keyboard without a Scroll Lock key, the on-screen keyboard built into the operating system has one, and it toggles the same state.
Num Lock on a compact keyboard
Many laptops and compact keyboards overlay a number pad onto the letter keys and, on some models, onto the arrows. With Num Lock on, those keys send digits instead of arrows.
Press Num Lock once and retest. The tester makes this unambiguous: it names the key that actually arrived, so you can see whether the board is sending Numpad4 where you expected ArrowLeft.
Other causes
- On a 60 percent keyboard, the arrows live on a layer and need Fn. If the layer does not engage, check the firmware rather than the switches.
- Gaming software or a macro tool may have remapped the arrows, sometimes as part of a profile that only loads for certain applications.
- Sticky Keys and Mouse Keys, the accessibility feature that turns the number pad into a pointer control, can both intercept arrow input.
- A dead arrow cluster with everything else fine is a plausible matrix fault, since the arrows often share a row or column with keys you rarely press.
Arrow keys that work everywhere except one program
If the arrows light up in the tester but do nothing in one application, the keyboard is not the problem and no amount of cleaning will change anything. Something is intercepting the key before that application sees it, or the application itself has bound the arrows to something else.
Games are the usual culprit, because many bind movement to the arrows and then lose the binding when a controller is connected. Text editors and terminal applications are the other common case: a modal editing mode, or a terminal in an application mode, changes what the arrows do without changing anything about the keyboard.
Remote desktop and virtual machine windows are worth checking too. Arrow keys are frequently captured by the host or the client rather than passed through, and the symptom is an application that ignores them entirely while everything else types normally.
If the arrows move the mouse pointer
That is Mouse Keys, an accessibility feature in both Windows and macOS that turns the number pad, and in some configurations the arrows, into a pointer control. On Windows it can be switched on accidentally with a shortcut involving Left Alt, Left Shift and Num Lock, which is easy to hit by mistake.
Turn it off in the accessibility section of system settings. Nothing about the keyboard needs touching.