Philips Hue 'Light Not Reachable' — How to Fix It

What it means

'Not reachable' (or 'Unreachable') under a Hue light means the Bridge can't talk to that bulb right now.
The bulb has to have power at the wall — a switched-off light switch is the number-one cause — and it has to be within Zigbee range, either of the Bridge or of another mains-powered Hue light passing the signal along.
One bulb unreachable is usually power or distance; lots of bulbs unreachable points at the Bridge.

Affected Models

Philips Hue White bulbs (A19, B22, GU10, E14, etc.)
Philips Hue White Ambiance bulbs
Philips Hue White and Color Ambiance bulbs
Philips Hue lightstrips, Go, Bloom and Iris
Hue Bridge v2 (square) — and the discontinued v1 (round), now unsupported

Common Causes

The wall switch feeding the bulb is off — a smart bulb needs constant power
Bulb too far from the Bridge with no mains-powered Hue light in between to relay
Bridge needs a reboot
Bulb in a metal fixture or recessed can that blocks the Zigbee signal
Recently removed a mains-powered Hue light that was relaying the signal, leaving a gap
Zigbee interference (a crowded 2.4 GHz environment, or a USB 3.0 device near the Bridge)
Bulb itself failing (rare, but it happens)

How to Fix It

Check the bulb has power.

Make sure the wall switch for that light is on and left on.
A Hue bulb is always 'on' at the socket — you control it from the app, a Hue dimmer, or voice; flipping the wall switch off just makes it unreachable.
If the switch keeps getting flipped, fit a Hue dimmer switch over it or use a switch cover.
Toggle the bulb's power once.

Turn the wall switch off for ten seconds, then back on.
That power-cycles the bulb and it often rejoins the network straight away.
Give the app a minute to catch up.
Reboot the Hue Bridge.

Unplug the Bridge for ten seconds and plug it back in; wait for all three lights on top to come back.
If several lights went unreachable at once, this is usually the fix.
Mind the mesh — don't strand a bulb.

Mains-powered Hue lights relay the signal for ones further out.
If you removed or unplugged a Hue light, a distant bulb can lose its path back to the Bridge.
Add a Hue bulb or plug somewhere between the Bridge and the stranded one, or move the Bridge more central.
Keep the Bridge away from large metal objects and USB 3.0 hard drives, which throw off Zigbee.
Re-add a stubborn bulb.

If one bulb stays unreachable after all that, in the Hue app go to add a light and search again with the bulb powered and within a few metres of the Bridge.
As a last resort, search by serial number (printed on the bulb).
If it won't rejoin even right next to the Bridge, the bulb has likely failed.

Frequently Asked Questions

Why does my Hue bulb keep showing as unreachable after I switch it on?

If it's reliably unreachable until you do something at the wall switch, the bulb is being cut off from power when that switch is off — a smart bulb can't be controlled from the app while it has no power.
The fix is to leave the wall switch on permanently and control the light from the Hue app, a Hue dimmer switch, or voice control.
If the bulb is unreachable even with the switch on, it's likely too far from the Bridge with no other mains-powered Hue light nearby to relay the signal.