If you cannot see “My Planner” in your default workspace, it might have been archived. Here’s how you can check and restore it:
Go to your default workspace.
Click on the “All” option at the top of your workspace.
Go to Archived and check if My Planner is there.
Click the three vertical dots on My Planner to open settings.
Select Unarchive.
Once you do this, My Planner should appear back in your default workspace.