I was trying to move the gears from park to reverse, and I think I popped a belt or something in the transmission.
The transmission does not have belts. It does have internal metal bands. These bands hold clutch drums to get the different gears to engage. If the band breaks, the transmission will feel like it went into neutral or park. Transmission removal and disassembly is necessary for an accurate diagnosis and for repair. This is something I suggest to be done by a qualified mechanic.