Welcome, @Kelly_Custer, and sorry to hear about this.
First, maybe some tasks remain but are “orphaned.” When a project is deleted, a warning dialog appears and notified your assistants that project tasks would be deleted except those that are either 1) assigned, or 2) multi-homed to another project. Since you’re asking about “my previous assistant’s assigned tasks,” maybe they’re still in your Asana. Use Search > Advanced search > Add filter > More > Deleted > select the deleted box, and also add other criteria like your assistant for the assignee field, and anything else. each task can be restored individually in the task detail pane with the Undelete button.
For everything else, you could ask Support if they can retrieve anything. Be as specific as you can. I’m not sure they can help, but perhaps worth a try. Temporarily, you can reach support as described here:
Thanks,
Larry