We have seen a few cases where Raspberry Pi Pico suddenly stops working. Sometimes the code stops running, the LED stops lighting or you can't see files when you connect your Pico to a computer.
Luckily there is an easy fix for this! On the Raspberry Pi Pico Getting Started page, there are instructions on how to reset your flash memory.
There is also a download button for an UF2 file that will fix this issue. You just need to download it and drag it from storage mode into your Pico file. This should reset it and bring it back to life.