The attached screenshot illustrates my problem, which is that I cannot run most of my jupyter notebooks that were working just a few days ago. Apparently, the directory where the modules are installed is no more in the search path.
For now, my fix is to add the missing path manually:
import sys
sys.path.append('/home/ubuntu/.lambda/lib/python3.10/site-packages')
But I would like to find a better solution, which does not require modifying every notebook file I use.