No, you do not lose copyright protection. You might receive less benefits when successfully enforcing your copyright, but you do not lose copyright protection. Today, if no source can be found, a publisher has to refrain from using that image/content and find something else that he may use.
By the way, this is being changed right now. The US is currently looking at the "Orphan Works" law that will greatly "improve" the way so-called orphaned works can be used by publishers. Orphan works are works that do not carry information about the creator/copyright owner. This will harm the entire content industry big time.