A lamp has stopped responding
The most common cause, and the order to check things in.
Check the lamp first
If the button on the column still changes the light, the lamp is fine and the problem is the network. If the button does nothing, the problem is the lamp.
The usual cause
A router that has changed channel or been replaced. The lamp holds its credentials but not its channel, and will rejoin on its own within about ten minutes.
Forcing a rejoin
Hold the button for eight seconds until the head pulses three times. That drops the network without unpairing, so you do not lose scenes or automations.