Your copyright is “infringed” when some one uses your music without your permission and the use is not a fair use, does not come within the scope of another exemption or a compulsory license. The law allows you to recover an amount equal to the sales you lost on account of infringement. If you are not able to calculate lost sales, you might be able to elect to take an amount fixed by the law. However, to be able to do this, you must register your work within 3 months of publication. In addition, you might also be entitled to a court order prohibiting the infringer from using your work.