The first thing you want to do is make a medical benefits claim or a PIP claim under your car insurance policy. Despite the fact that you did not cause the accident, your medical expenses from the car accident are covered by your own car insurance company, up to the limit you purchased. The minimum medical benefits coverage in PA is $5,000. Therefore, you want to make sure that you are submitting your car insurance policy information to medical providers. See Pennsylvania Car Accident Injury Law - Medical Benefits Claims
In addition to filing a PIP claim with your own car insurance company, you may be able to file a lawsuit against the truck driver for your damages. Your right to sue is dependent on whether you have limited tort on your car insurance policy and on the extent of your injuries. For more information about limited tort and your right to sue, see Pennsylvania's Limited Tort - When You Can Sue For Pain & Suffering (Part A)
However, from the information you gave, it seems that you may not be able to identify the truck because the truck was ahead of you and the truck driver may not have stopped or known about your accident. In such a situation where the truck cannot be identified, you may be able to file an uninsured motorist claim pursuant to your car insurance policy if it can be established that the box came from an unidentified truck.
If the box was physically placed on the road by a person or rolled onto the highway from a hill adjacent to the highway, you would not be able to file an uninsured claim because the accident did not arise out of the ownership, maintenance or use of an uninsured vehicle.
Pennsylvania’s car accident law on uninsured motorist claims is very complex, and it is important that you speak to an experienced PA car accident lawyer. Feel free to contact me at 877.944.8396 for a free initial consultation.