Welcome back!

If you had a 3rd retainer it's possible it's subscription is suspended - this is because players only get two free retainers by default, and any extra retainers after that require you to pay a regular subscription for each extra retainer. If this subscription lapses or is suspended, then you lose all access to that retainer until you reinstate it's subscription. Don't worry though if you've lost your items, everything stored on that retainer remains safe even if it's suspended.

To check if you have a third retainer and if it's subscription is lapsed, login to the Mogstation and on the main page for your account, it will have a section that says 'Additional Retainer'. Under this, if you have any active retainer subscriptions it will list it as 'subscription active', but if it is not, it will simply say 'inactive'. If it's the later, click on the 'Your Account' button, and then click 'Retainer Service' - this will allow you to reactivate suspended retainers, or change how it's paid for.

Hopefully this has helped.