Donald Trump's second impeachment trial ended on Feb. 13 with acquittal. A Senate majority, 57 to 43, voted to convict, but that requires 2/3. The House impeached Trump one week after a Trump-incited mob attacked the U.S. Capitol, murdering a police officer, injuring many, and seeking to murder Vice President Mike Pence, Speaker Nancy Pelosi, and other lawmakers.
Trump left office on Jan. 20 after the House impeachment and before his trial. That is not a bar to impeachment; impeaching officers after their time in office was the norm in Britain and in the Colonies at the time of the Constitutional Convention. In 1876, former Secretary of War William Belknap was impeached after leaving office. The ability to impeach after leaving office has two good effects. First, it means there is no "last month" exemption of an official from laws or duties. Second, our Constitution allows, if impeachment results in conviction, for the Senate to bar the convicted person from holding federal office again.