In terms of what was known before, say, the 18th century, there were very limited sources of what we'd normally consider rather dim light, but fire wasn't the absolutely only light source (not counting the sun, moon -- as it was thought to give its own light in pre-Renaissance times -- and stars).
Glowing insects were the commonest, and brightest. Fireflies can be nearly bright enough to (briefly) read by, if you can get even two or three of them near your page (their tendency to blink in synchronus is both helpful, because they're brighter, and unhelpful, because it'll be bright then completely dark). Firefly larvae are "glow worms" -- and I'm not certain whether they glow steadily or flash like fireflies.
Other sources of phosphorescence, like certain fungi (foxfire), are very faint -- easily bright enough to see in a dark forest, but not really bright enough to see by.
I can't say whether it was known in pre-scientific times or not, but the mineral sphalerite (zinc sulfide) is moderately common in some regions. It's the material that's incorporated into "glow in the dark" paints and plastics -- it will absorb light energy and then release it over time, though it can't be "kept" and only lasts minutes to an hour or so unless there's a source of either UV radiation or something else (this was used in radium clock dial paint, excited by the radiation from the actual radium).
Pure phosphorus was made by alchemists well before the 18th century, and glows from slow oxidation even when stored in water -- but I'm not sure this wouldn't be a form of the lost fire.
Another possibility is electric light. There is (slight, controversial) evidence that electricity had been created during the Bronze Age -- the Baghdad Battery was a wine jar that contained a rolled core consisting of dissimilar metals separated by fabric; it would have functioned as a simple electric cell if filled with wine or vinegar. Speculation is that it was used for electroplating, to allow dishonest jewelers to pass off cheap base metal goods as solid precious metal. However, there are various ways this low voltage electricity could be turned into light without requiring the ability to produce tungsten or platinum alloys (to take the heat).
One possibility is electroluminescence; this requires 100 Volts or so (which is a big bunch of battery jars), but could be made from minerals and metals that were available before the 18th century, if someone knew how (or stumbled on the effect -- say, a jeweler plating a ring that already has a tourmaline mounted).