For all delivery-related issues, contact your local post office / courier to make inquiries using your tracking number as soon as you can. If your parcel is supposedly signed for, do also check with anyone who might have access to your apartment or lives nearby to see if they signed for your parcel while you were away.
Do not wait for the post office to contact you! I have had returned packages from customers because the postmen don’t leave a notice for failed deliveries and the package was returned after the claim period. Please check the tracking status of your package regularly until you receive it.