Copyright registration is not required for a work to be protected. A work is protected at the time of its creation, and a copyright symbol is not necessary. Copyright registration, however, is necessary to pursue an infringement case. This can be done retroactively through the U.S. Copyright Office. The cost is $35 if done online and $65 on a paper form.
 U.S. Copyright Office, Copyright Notice, Circular 3 (2011), available at http://www.copyright.gov/circs.
 U.S. Copyright Office, Copyright Office Fees, Circular 4 (2011), available at http://www.copyright.gov/circs/circ04.html#fees.