I went to the place I had reserved to stay at, but the person in charge was not there and I couldn’t contact them or get the key.

Check the details listed on your booking confirmation again—the location and time scheduled for the key transfer to take place. Also, call the number listed on the booking confirmation for the manager or person in charge at the lodging, and explain the situation. If you made the reservation on a travel website designed for booking accommodations, check the website’s terms of service to see if the website offers any remedy for when a problem occurs. Keep in mind that even if a lodging has “Hotel” in the name, it may in fact be a guesthouse or private residence temporarily taking lodgers (minpaku). At such lodgings there is no front desk, and keys may be transferred using a key case positioned near the door, so be forewarned. If you are unable to contact the designated person and are unable to get into the lodging that you booked, make note of the time you arrived, the time you called the person, and other details of the situation that day so that you can explain everything later to a third party. It may also be a good idea to take a picture of the entrance to the lodging as a record. On that basis, later explain the arrangements for the lodging and what happened that day, and negotiate for a refund of the lodging fee, etc.