Keokuk at Charleston.
Maybe one of the Cairo-class to plunging fire, iirc?
The FAQ on the linked site answers that, I believe.
2. How many ironclads were sunk in action?
Very few. The normal fate for a Southern ironclad was to be destroyed to prevent her capture. On the coast, the USS Keokuk, a bizarre little non-monitor design, was sunk as a result of gunfire from the attack on Fort Sumter in April 1863; the only other ironclad to be sunk by gunfire was the USS Cincinnati, a riverine gunboat, by fire from Vicksburg, but she was raised and put back into action shortly afterward. A much more deadly threat was the naval mine ("torpedo" in the terms of the time); the USS Cairo, a sister of the Cincinnati, became the first warship in the world to be sunk by a mine on 12 December 1862. The Union monitors Patapsco, Tecumseh, Osage, and Milwaukee were all sunk by mines, as well as the Baron de Kalb (ex-St. Louis), and the Confederate Albemarle was sunk by a spar torpedo mounted on a steam launch.
Ramming was also quite effective, especially in the constricted Western rivers; Southern rams claimed the Indianola, Mound City, and the often-unfortunate Cincinnati, though the latter two were raised and put back into action. So, of a total of sixty-six ironclads on both sides combined, eleven were sunk in action (as noted, the Cincinnati was sunk twice).
Doc adds: ironclads almost always sank in rivers or coastal waters and could be raised. The Confederates raised the guns from the KEOKUK and added them to their defenses at Charleston.
