Many, many attempts dating back well before World War II.
As far back as 1938, the Oster Conspiracy aimed to overthrow Hitler if Germany went to war with Czechoslovakia over the Sudetenland. Their aim was to remove Hitler either by having him arrested or killed and restore the monarchy.
In the end, after the Munich Conference with the connivance of France, Italy and the UK, Hitler seized the Sudetenland without a war. He had massive popular support as a result, so the plot fizzled out.
