You need to check your MongoDB configuration file, which I’m not sure where’s located, since you haven’t listed what version of Ubuntu your upgraded from and to.
Usually the database is located in /var/db/mongodb or /var/lib/mongodb

This command should list the path of the databases, if your configuration is in /etc/mongodb.conf

grep dbpath /etc/mongodb.conf

Which version of MongoDB are you running and which version of Ubuntu (both before and now)?