A planet will "die" generally from one of the following "natural" fates:
Death by Cooling
The planet core slowly cools. The planet then loses its magnetosphere, and solar winds begin stripping the planet of its atmosphere. This is likely what happened to Mars, and what is currently happening to Venus.
This is a very slow process, happening over millions of years. When the planet is completely cool, it will not likely be able to sustain life (due to lack of atmosphere), but will otherwise be the same: a cold hunk of rock in space.
Death by Star
As a star ages, it gets bigger and brighter and hotter. For Earth, this means that we will be outside of the habitable zone of the Sun in 2-3 billion years. The brighter star will slowly cook the planet, killing everything on it.
As the star continues to grow, it may extend past the orbit of a planet, and draw that planet directly into the star. The Sun will likely expand beyond the orbit of Venus, but it may not quite reach Earth (making Earth the new Mercury).
If the star is massive enough (about 8-12x the Sun), it can go supernova. The supernova would completely disintegrate all objects in its solar system, and would be visible tens of thousands of light years away. However, before a star can go supernova, the slow death by absorption or overheating would already have happened.
Death by Impact
A large object, such as a planetoid or asteroid, may impact a planet. Depending on the size, this may lead to an extinction event, wiping out all life on the planet, or it may literally rip the planet apart.
A leading theory for the formation of the Moon is that a Mars-sized planetoid smashed into Earth, creating the debris that become the Moon. However, even in this scenario, the Earth is still here. It would not destroy the planet so completely as to prevent it from reforming.
Death by Gravitational Perturbation
An mass, such as a rogue planet, large Oort Cloud object on a weird orbit, or very nearby star, may provide enough gravitational force to perturb the orbit of the planet in question.
For example, a galactic collision may rearrange the star systems, putting two or more stars in close proximity, or sending large objects (like planets) into other star systems.
In the simple case, the planet is moved to a different orbit. This may move the planet close enough to a large planet to be torn apart via tidal forces. This won't happen super quickly, but it could completely destroy the planet, making another asteroid belt.
In the extreme case, the planet may either be sent deep into space, where everything on it dies due to lack of sunlight, or it is sent into the sun, where it burns up (see above).
A couple notes:
unless the properties of a planet are significantly changed by manual intervention, a planet will never:
- Fall apart
Planets do not liberate energy from matter (otherwise, they would be stars), which is required for the first two. The matter in planets is bound by gravity, which is always present, and that's what prevents the third one.