We were whatching a movie and our smart tv just went black.

If the sound is on then the most likely problem would be a loss of power to the screen backlight. This is the light source that illuminates the screen. Often this is an array of LEDs. There is a circuit that supplies the power to them and there is another circuit that regulates the power for the brightness control. You need to take it to an experienced technician to determine the problem and fix this.