By 1971, the U.S. was struggling to maintain the proper supply of dollars to adhere to the gold standard. It also faced deficits that led to foreign-held dollars that exceeded the U.S.'s own supply of gold, meaning it could not redeem dollars for gold if the international supply had been called.
As a result, Richard Nixon ended the dollar's convertibility to gold, effectively ending the gold standard. At this point, it seems unlikely to return since most countries prefer to give central banks control of the money supply and monetary system.