sessions on colab get recycled pretty quickly if you leave them idle which means all the files vanish. when you come back it automatically reconnects but you’ll need to re-run previous cells which your current cell depends on. in your case probably just re-run everything from the top.
Are you working on the cloned directory? It shows the path /content/images/ as the directory where the grizzly.jpg was not found. Maybe you have cloned the github repository but aren’t working where the grizzly.jpg is present.
If you have cloned the course-v4 on the google drive i.e. within content/My Drive/course-v4, then you need to go there to access the file or you could copy the course-v4 directory to content/ to access the files where you are right now.