Welcome to the Asana Community Forum @Renata_Miskei-Sebok!
Usually when a user is removed via the admin console you have the option to create a project with all the tasks of this user (including completed ones) that you can then reassign if required.
Here is a great post on what to consider doing before removing a user: Remove/Offboard/Deprovision a User from an Organization, and what to do beforehand
Now in your case it seems the user was removed already, have a look here this should help: Recover tasks assigned to deleted account - #2 by Emily_Roman