The most important question is, what is that machine's warranty status? It sounds as if it has probably broken rather than fallen apart, and since the machine is nearly new, it isn't the ideal example to practice on if it is under warranty.
To get anywhere with discussing the problem, we would need some pictures of the machine, including specific shots of the affected parts.
For your general information, there are usually two engine-driven belts: one drives the cutter and the other the transmission, and thence the back wheels. The cutter system is mounted on a shaft and bearings attached to the deck. The bearings support the shaft and enable it to rotate. At the top of the shaft near the bearings is a V-belt pulley, attached to the shaft. Below the pulley, also attached to the shaft, is the blade disc, which is simply a plate to which the blades are attached. The shaft, pulley and blade disk are all rigidly attached together. It is unclear from your description where the separation has occurred - it sounds as if either the blade disc has broken where it attaches to the shaft, or the shaft's bearing housing has broken out of the cutting deck, leaving the whole shaft subassembly detached from the mower. If that happened many bad things would occur, including destroying the belt, but the main problem would be the damage to the deck and bearing housing, which are expensive parts.
