@Doomguy666:
A very slow cranking (or at least slower than usual) is a sign the battery is on its way out; average battery life span is about 5 years.

I will always advise when the battery is testing as weak, replace it. A weak battery will overwork the alternator, shortening its lifespan.

A no crank, no start (or sometimes one single, loud 'click' with no crank) is a possibility of a starter. A really simple test is to have the headlights turned on, and somebody watching the lights while the key is turned to the crank position; if the lamps dim, but nothing happens (dead silence), it is a good sign that the starter is kaput.

Dead silence throughout a key cycle is a tougher one, as it could be many things, such as ignition switch, a relay, et al.

Cars suck, sometimes. The trick is to always be listening to them; vehicles are constantly giving us updates as to their condition, we just need to pay attention to changes in their behaviours.

@Sharp_Coyote: When I turned the key the dash lights flickered and it didn't turn over at all, than there was an intermittent clicking.
I didn't notice anything different the day before it died.

The mechanic said the battery was seven years old, so quite a bit older than the average.
I really should learn more about cars because I was at a loss when it didn't start.