No, it's never happened. The US Government, was set up so that too much power was not in any one group's hands, so to prevent "Tyranny" or as we would call it today a "Rule of Force" from a Dictatorship.

The 3 divisions of government, the Legislative, the Executive and the Judicial are established as separate so that one branch can't control the actions of the nation/government like in a dictatorship.

Congress could impeach the sitting President (and if they wanted to, could actually force the President out). Then the Executive Branch would pass through the line of succession.

Congress is suppose to approve the Cabinet nominations of the President. It was never meant to be just on 2 party lines, which makes actual scrutiny questionable. If individuals in the Legislature or the Executive Branch have broken the law the Judicial Branch is suppose to enforce the laws. If the laws are made so as to affect the rights of all humans as set forth in the Constitution, then it's the Judicial Branch's job to over turn those laws to protect our rights.

If the entirety of the Federal Government was seen as failing and was structurally unsound the Constitution of the US can be amended, which it has been. There are 2 ways to amend the Constitution, one is through legislation from Congress, that the States Ratify by vote of the people, OR a under Article 5 of the Constitution, a"Convention of States" which would be like a "Constitutional Convention" and the States approve to send representatives/delegates to an official Convention, which can make as many changes to the Constitution as the decided. This has never been done yet.

The closest it's ever come was the impeachment of Richard Nixon over the Watergate break-ins, & cover-ups. He is the only President to resign from office. Bill Clinton was also impeached for lying under oath to Congress. Congress, never pursued the matter to remove him from office.

Further reading on the Impeachment of Nixon: http://www.historyplace.com/unitedstates/impeachments/nixon.htm