What happens if our Sun suddenly becomes a black hole? I already know that the mass of the Sun is too small for it to become a black hole; I'm asking for a hypothetical scenario.

First, it is unlikely that our sun would become a black hole as it is not sufficiently massive. Instead, it is likely to expand into a red giant and then gradually fade away to become a white dwarf star, gradually decreasing in mass and energy. A star massive enough to become a black hole would radiate so much energy and heat that anything as close as earth's orbit would be uninhabitable.
The way in which a star evolves into a black hole is generally by first becoming a supernova. If one bent the laws of physics to imagine this happening to our sun, the catastrophic release of energy would vaporize the earth, and so no living beings would be around to observe the process. Next, if sufficient mass is left over after a supernova, the core of the star gradually shrinks until it becomes a black hole. Although no energy passes the event horizon of the black hole itself, material nearby can be very hot and energetic and emit large amounts of radiation, including x-rays.
To imagine some sort of science fiction universe in which the sun was instantaneously replaced by a black hole without going through the supernova phase, one would still end up with something that would eject streams of high energy particles and radiation, meaning that all living beings on earth would die with minutes or even seconds.
If, for example, there were a technological civilization on a planet orbiting a nearby star such as Alpha Centauri, several years after the sun became a black hole, their scientific instruments would observe that the sun would no longer resemble a star, but rather a turbulent gas and dust accretion disc with a black hole at the center. If they looked at stars in the same region of the sky, they would notice the phenomenon of gravitational lens effects, in which light would be bent by the gravity of the black hole.
https://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-a-black-hole-58.html