Black holes are formed two ways:
Massive stars dying, and 2 neutron stars merging.
For massive stars dying, they spend most of their lives fusing hydrogen into helium. This creates a tremendous amount of energy. Our yellow dwarf star (known by its more boring name, the sun) converts 600 million tonnes of hydrogen into helium. A star is balanced between the radiation it emits and gravity. With stars with much more mass than our sun, the temperature and pressure allows them to fuse helium into lithium, fusing it into carbon, oxygen, neon, silicon and then iron. Iron cannot fuse into another element, and so iron builds in the core until it hits a critical amount. The radiation to gravity is broken, the core collapses and then the star dies in a super/hypernova explosion. This leaves a neutron star (will be done in another thread) or if the star is large enough, the core collapses on itself into a black hole. If you looked at it, you would not be seeing the black hole, but the event horizon. Anything that crosses, needs it's speed to bypass c (300,000,000 km/h) which is impossible under the laws of physics. The event horizon is completely black, so you do not see anything. If that's the black, what's the "hole" part?
The singularity.
Neutron stars merging, this doesn't just happen after midnight, they merge over millions of years and then release a gamma ray when they merge, and then fall under their own gravity.
(Credits to Kurzgesagt – In a Nutshell and Wikipedia for the info)
Coin (spoilers)
Usually black holes will form after a giant star explodes. Let's assume your nickel was just 5 grams.
But is the black hole weighing 5 grams, or is it the size of the coin?
Short answer: You die.
Long answer: Depends.
If it (the black hole) weighed 5 grams, comparing it to the smallest atom (hydrogen atom) would be like comparing a human being to UY Scuti. Absolutely tiny. The black hole's radius would be 10^-30 meters, a hydrogen atom is 10^-11 meters. The black hole would also evaporate all it's mass due to Hawking radiation in a very short period of time (10^-23 seconds), and the 5 grams would be converted to 450 terajoules of energy, producing an explosion three times bigger than the atomic bombs dropped on Hiroshima and Nagasaki combined.
In the end: You die. You also lose the nickel.
If it was the radius of the nickel? It's mass would be far bigger, possibly the mass of Earth. It'd have a surface gravity a quintillion times stronger than on Earth. You'd like to think it would just rip you apart and then continue living there, but no. It sinks to the centre, not producing a hole but with matter falling into the black hole. The black hole then slowly consumes the Earth, with the Earth orbiting the black hole as if it was it's new star, all having tonnes of mass eaten with each pass. The Earth then collapses into a disk of hot rock, essentially killing us all. The moon's orbit is affected, turning highly elliptical. The Solar System itself? Not much is changed, other than the asteroid belt being thrown around and the planet orbits slightly changed.
Also, black holes do not suck things up like a vacuum cleaner. If you swap the sun for an equally massive black hole, nothing would change for Earth other than we'd freeze to death.
(Again, credits to kurzgasagt for the info (god i'm bad at german names)
