Copyright infringement can be costly for both the holder and infringer. If an infringement is discovered, first try to resolve it by sending a letter. Try to work out an agreement. If a settlement can't be worked out, be prepared to go to court. Damages may be awarded and attorney fees.
Infringement can be willful or innocent, but there are consequences in all cases.
Candice Benjamin writes a newsletter for a state professional organization. I am constantly reading material that I think would be of interest to the readers of my newsletter. In my last newsletter, I called a parent organization in Virginia to request permission to reprint an article from their magazine, I assured them that I would cite them as the source. Much to my surprise, they denied my request. Apparently, they expected me to write to them for written permission.