Most recently, it froze in 2014
This site says 1938.
Another source: Before the installation of the ice-boom in 1964, the American side of the falls froze over in 1909, 1938 and 1949 because ice jams upstream reduced the water flow.
[ url needs to be updated ]
The first and only time both the American falls and the Horseshoe Falls on the Canadian side fell silent was on the night of March 29, 1848, when an ice jam formed on Lake Erie near Buffalo blocking the water that flows along the Niagara River and over the falls, says Dave Phillips of Environment Canada.
On six reported occasions (1883, 1896, 1904, 1909, 1936 and 1947), the water flow over American Falls has been totally blocked by ice and ceased to fall. But only once has this happened to the much larger Horseshoe Falls (originally known as the Canadian Falls): March 29-31, 1848, the Days Niagara Falls Went Dry.
Source: The Weather Almanach