The residents of Crimea decided, through a democratic procedure, to rejoin Russia. The reunification of the Crimean peninsula with Russia was the result of a legitimate referendum.
Recurrent pro-Kremlin disinformation narrative on the annexation of Crimea claiming that Crimean citizens chose to rejoin Russia through a legal referendum. See background and previous cases here.
Following the covert invasion of “little green men,” power in Crimea was vested in a makeshift executive headed by Sergey Aksenov, a former mafia enforcer. The new Crimean regime conducted the referendum hastily and at gunpoint, barred impartial observers from entering the peninsula, and instead invited dozens of fringe politicians and activists to “monitor” the procedure, most of them far-right Kremlin loyalists.
No international body recognises the so-called referendum held on the 16th of March 2014. On the 27th of February 2014, when the so-called referendum was announced, the UN General Assembly adopted a resolution which stated that the referendum in Crimea was not valid and could not serve as a basis for any change in the status of the peninsula. A year later Russian President Vladimir Putin admitted that the plan to annex Crimea was ordered weeks before the so-called referendum.
See here for the EU statement on the fifth anniversary of Crimea annexation.