Ft Donelson was a Confederate fort on the Cumberland River in northwest Tennessee. On February 12th, General Grant and about 15,000 Union troops began to surround the Fort. Then, they waited for gunboats and reinforcements to arrive.
Two days later, 4 heavily armored Union City-class Ironclad Gunboats approached the Fort along the river, and tried to breach its heavily fortified Eastern border. The Fort's water batteries blasted them mercilessly, eventually driving them back up the river without their steering mechanisms.
The soldiers in the Fort then attacked the Union soldiers on their right flank, and both sides suffered over 1,000 casualties. After the right flank ran out of ammunition, General Grant ordered the left flank to assault the Fort, and they were able to make considerable progress.
By the next morning, General S.B. Buckner, commander of the Fort, sent out a message of surrender.
You can read all about this in detail in the New-York Daily Tribune article, dated February 24, 1862. The article is split into a top portion and a lower portion.