Seems like the viable theories would be: a) something changed in the arrangement of the files on your Google Drive or b) you’re not actually in the same “current working directory” in the active notebook on Colab.
Might be worth having a look through Saif’s excellent tutorial on how to get course notebooks to work in the Colab context. Note that the material there is specific to the DLS course notebooks and I have not looked at any of the newer “short course” assignments, so not sure whether there are any significant differences in the notebook you’re actually working with.