You do the a desperate hand dance where you tap the flashlight with you palms, shake it like maracas, and finally stare into the LEDs for answers that aren't there.
And frequently, it's not the shaking or the tapping or the clicking or the tightening of the lens cap or any of the other actions which remove the corrosion, oxidation, or misalignment and reestablish an electrical between the batteries, spring, switch poles, and bulb.
It's the gentle, inquisitive wiggle as you stare directly at the bulb which causes it to turn back on.
For related reasons, never look down the barrel of a gun which has not been sufficiently disassembled. There are no answers there.
