The delivery company will leave a message in your mailbox which states that a delivery attempt was made. You can also find this information then on the tracking link. This message will contain instructions on how to contact the shipping carrier. Normally, there is a second and third delivery attempt foreseen in the next days. After 3 delivery attempts, the order will automatically be shipped back to sender.
In some cases, your order will be delivered at a local post office or parcel shop after the first delivery attempt.