A significant risk for the U.S. nuclear stockpile is for a weapon to accidentally detonate or go missing. A weapon can go missing for a number of reasons-it can be misplaced, or lost when the vessel carrying it crashes, for example. Such an event is called a "broken arrow" incident and is the most serious type of military nuclear incident short of nuclear war.
Perhaps most important to note at the outset is that there has never been a nuclear blast resulting from a broken arrow incident. Nuclear weapons are designed not to detonate accidentally - the exact technology has changed over the decades, but especially at first, the technique was to keep the nuclear material separate from the detonation explosives until just before the nuclear explosion is meant to happen. 
Another important note is that the locations of nuclear weapons are generally not disclosed by the military, for obvious reasons, so broken arrow incidents can take years after they occur to come to light. For example, in 1965 a hydrogen bomb was lost in the Pacific Ocean when the plane carrying it fell off the flight deck of a U.S. aircraft carrier that was returning from Vietnam . The bomb was neither recovered nor publicly reported lost until 1989, because the U.S. did not want the word to get out that nuclear weapons were part of the Vietnam War plans and because the loss happened near a Japanese island.  Of the 32 broken arrow incidents acknowledged as such by the Pentagon between 1945 and 2007, 7 were not reported at the time of the incident and 18 were not originally reported as involving nuclear weapons. 
There are a number of documented broken arrow incidents that have occurred since the Cold War, but they have become rarer as time progresses. One thing to keep in mind is that the U.S. nuclear arsenal was larger during the Cold War than it is today; in peaked at over 61,000 warheads in 1966 and stands at 5,113 today.  Another thing to keep in mind is that during the Cold War, many nuclear weapons were used as part of the Strategic Air Command (SAC). This program placed warheads on bomber planes and flew them around as a nuclear deterrent to the Soviet Union, but having weapons mobile on airplanes obviously opens them up to additional risks and indeed, in 1968 a B-52 crashed in Greenland with four 1.1 megaton nuclear bombs on board. One of the seven crew members died, the jet fuel on board exploded when the plane crashed, the conventional explosives on the nuclear warheads detonated, and radioactive debris was scattered around the crash site. 
That is not to say that, even with today's reduced nuclear arsenal and technological and logistical advances, missing nuclear warheads are a thing of the past. A less-serious designation, "bent spear," refers to incidents in which warheads are out of official control but there is a less-serious risk of their leaking radiation or being seized by non-U.S. military parties. A bent spear incident famously occurred rather recently, in 2007, when a series of checks failed in a missile transfer and six nuclear warheads were accidentally installed on a B-52 and flown about the country under relatively low security (those involved thought they were conventional warheads) for 36 hours before the error was caught and corrected. 
As one might imagine, a serious investigation followed and found a series of failures in the missile-check procedure. Similarly, investigations following other incidents led to changes in nuclear policy, although omissions in the records available to the public make it difficult for a civilian to piece together a comprehensive picture of all broken arrow and bent spear incidents, and other nuclear close calls (false attack alarms during the Cuban Missile Crisis, for example). 
With 32 broken arrow incidents on the books, one can reconstruct a number of stories like the Greenland crash summarized above. Going into detail on all 32 would be (and has been) the subject of entire books, but a few conclusions stand out. First, broken arrow incidents are very serious and are investigated as such by the military, perhaps to the point that not all information on them is publicly available. Second, broken arrow incidents have empirically become rarer since the Cold War, with the last one taking place in 1980. Last, maintaining nuclear security remains an active pursuit, since the bent spear incident of 2007 shows that complacency will eventually lead to missteps.
© Katie Malone. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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