This sounds like the plot of a book I read years ago called "Inherit the Stars" by James Hogan. In it astronauts find a body on the Moon in a space suit that is totally foreign. This launches an investigation which reveals that the body is millions of years old and was left there by the inhabitants of this 5th planet who were on the Moon of the 5th planet when the planet was destroyed. The moon then floated through space until it was captured by Earth's gravity. The survivors then went down to Earth to become our ancestors. Great Book and the first of a series.
The asteroid belt has an estimated total combined mass of less than 1 tenth of the Earth’s moon. Which pretty much rules out that it is an "exploded" planet.
Also Jupiter has a profound effect on the asteroid belt.
Jupiter has a semimajor axis of 5.2 Astronomical Units, thusly its orbital period is 11.86 years. Since the asteroids are not all at the same distance from the sun, some of them will have an orbital period of one half that of Jupiter. This puts those asteroids in a 2:1 orbital resonance with Jupiter. The result of this resonance is gaps called the Kirkwood’s gaps.
So here is the rub, why did not these asteroids form a small planet in the first place?
The reason is the gravitational force of Jupiter. It perturbs the asteroids giving them random velocities relative to each other. Another effect of both Jupiter and the Sun on the asteroid belt is a group of asteroids that both precede and follow Jupiter in its orbit by 60 degrees. These asteroids are known as the Trojans.