If you tied 30 American school buses together and set them moving at a typical earth orbit speed, you would have 300 tonnes of matter moving at 10 km/s. A crash would equivalent to 3.5 kilotons of TNT. About 1/4 Hiroshima.
If instead of 30 school buses, you used one Ticonderoga class missile cruiser, the kinetic energy would be equivalent to 100 kilotons of TNT - about seven Hiroshima bombs.
A School bus has a mass from 4,536kg to 16,329kg. For easy maths, I took a middle number of 10,000kg. Thirty of them thus mass 300,000kg.
The Vis-Viva Equation allows one to to calculate the velocity at one point in an orbit, given the highest point of the orbit.
The radius of the Earth is the lowest point of the orbit, where the ship touches the surface. That is 6,371,000 meters from the center of the Earth.
Geostationary Orbit is 35,000,000 meters above the surface. To pick a number for "in orbit", I chose 10,000,000 meters above the surface, which means a highest point of 16,371,000 meters from the center of the Earth.
The calculator I linked to gives the "standard gravitational parameter" a value of 3.986004418e+14 m^3/s^s for Earth. Wikipedia provides a list for other planets.
The calculator then gives a velocity when the object touches the ground of 10,039 m/s, which I'll call 10 km/s.
We can then plug the number into a Kinetic Energy equation and get the number of Joules of energy that will be converted when the object comes to a crashing halt. Since we kept the numbers easy, the result is 15,000,000,000,000 Joules.
Wikipedia tells me that one kiloton of TNT is 4,184,000,000,000 Joules, so simple division tells me that the energy release on impact is 3.585 kilotons.
For the Ticonderoga cruiser, the only change is the mass, and Wikipedia says that the Ticonderoga masses 9,600 long ton. A long ton is 1.016 metric tonnes. That means a Ticonderoga masses 960 times more than one school bus, or 32 times more than 30 school buses, so the impact energy goes up by a factor of 32. That actually gives you 114 kilotons, which I rounded down to 100 kilotons.
And finally, the "Little Boy" Hiroshima bomb had an estimated yield of 15 kilotons.