Orphan works are works for which there is no clear copyright owner. Orphan works are protected under copyright law, as are all intellectual and creative works, but because of the lack of obvious ownership, it can be almost impossible to obtain copyright permission for orphan works.

If you have decided you need permission from a rights holder to use a copyrighted work, but can't locate any way to contact them, or never heard back from them after you contacted them, you might be dealing with an orphan work.

Unfortunately, a work's status as an orphan work does not allow you to treat the work any differently than any other work.

If you are in this situation, you must either,

1. Reevaluate fair use. If after a long search for a copyright holder or permission from a copyright holder you can find no way to get permission or pay a licensing fee, this may change the way you look at the "market effect" factor of fair use. The potential market for this content is unlikely to be harmed by your use of the content, and you may be able to more generously evaluate fair use for this work. 
2. Change the use of the orphaned content to fit within fair use. By using only a portion of, fewer copies of, or limiting access to the work, you might be able to change the scope of your project to fit within the boundaries of fair use.
3. Find a replacement. Is their a piece of content in the public domain, or with a more cooperative copyright owner that could fulfill the needs of you project as well as the orphan work?

To read more about Orphan Works and what else oyu can do when you encounter them, check out the resources linked below.