Any idea how long ago it was drained? It wouldn't hurt to fill her up with water and check for leaks. If it was dry and there are some leaks, let the water sit and the wood will swell. Once done mix up a solution of (1) ounce of sodium metabisulphite and half (1/2) ounce of citric acid per gallon of water. Do a few gallons of this and slosh it around in the barrel. dump, rinse. You're ready to go.
I store my barrels with the same solution above when not currently using. Keeps the nasties away and keeps the wood swollen.