Maybe. It depends on what they were doing at the time. As an invitee, someone on your property for mutual benefit is owed a great duty of care. This means you not only have to warn or make the property safe for them, you also have a heightened duty to inspect and can even be liable if you could have discovered a dangerous condition upon reasonable inspection of the property. However, horseplay or trespassing into other parts of the property can negate the duties owed or alter the classification of the injured person. You further need to take into account how they hurt themselves. For example: Did the ladder they used collapse? Whose ladder was it? Did you know of the ladder defect? How old is the ladder?, etc. You can see that premises liability law is large and complex, you should always contact an attorney if someone has been injured on your property.