The power plant would fall down. Some fraction of its walls, grounds, cooling water, stored fuel, and spent fuel would be volatilized into airborne dust.
The fuel assemblies would almost certainly not melt down, unless you make some very favorable assumptions about the type of power plant and the geometry of the blast damage. Fission occurs when nuclear fuel exceeds a critical density. Power plants generally operate by constructing a supercritical fuel mass with some amount of neutron-absorbing "control rods" interspersed. Meltdowns happen if the control rods are removed (as at Chernobyl) or if the reactor's heat can't be removed while it's operating (as at Fukushima). But if you just knocked the whole assembly down, it would tend to become less dense.
Nuclear weapons work by very carefully engineering the fissionable material to maintain its density for as long as possible during the blast, which has a lot to do with its shape, its surface, the geometry of the chemical explosives which bring the pieces together, and other secret technical details. You can't generate a high-power fission chain reaction by starting from a fuel assembly meant to operate in a controlled way and knocking it over.
Any vaporized fuel or waste would eventually fall back down out of the atmosphere, a phenomenon known imaginatively as "radioactive fallout." The waste would be more dangerous than the fuel.