Who is responsible for the loss caused by the billboard in the elevator?

Hello! In this case, ask the property management company, because there is a causal relationship between their failure to maintain the elevator and your injury.

Article 126 of China's General Principles of Civil Law stipulates that the owner or manager of a building or other facilities, as well as the shelving and hanging objects on the building, shall bear civil liability if they collapse or fall off and cause damage to others, except those who can prove that they are not at fault. Article 85 of the Tort Liability Law stipulates that buildings, structures or other facilities and their shelving and hanging objects fall off and cause damage to others. If the owner, manager or user cannot prove that he is not at fault, he shall bear tort liability. After compensation, the owner, manager or user, if there are other responsible persons, has the right to recover from other responsible persons.