Okay, imagine you're playing with your toys, and you have a really cool toy or action figure that you really like. But, later on, you can't find it anymore. It's nowhere to be seen. That's what happens with a lost literary work. It's like someone wrote a really awesome story, but then people can't find it anymore because it's been lost.
This can happen for a few different reasons. Sometimes the physical copies of the work get lost over time. Maybe the paper it was written on got thrown away or destroyed somehow. Other times, the work might get lost because it was never copied or printed in the first place!
Especially in the past, it was really hard to copy books or stories. People had to write everything out by hand, which took a lot of time and effort. That means that sometimes works would only have one copy, or even just a few copies. If those copies got lost, there would be no way to find the story again.
And sometimes even if there are copies of a work, they can be lost because people forget about them. Maybe they get tucked away in a library or archives and nobody looks at them for years and years. Then, when people finally do start looking for them again, they're nowhere to be found!
It's really sad when a literary work gets lost because that means we can't enjoy it anymore. We don't get to read the story or learn from it. But sometimes lost works can be rediscovered if someone happens to come across them. This is really exciting for book lovers and historians who might get to read a story that's been forgotten for a long time.