Q: Transmission not engaging

Transmission oil needs to be changed at the correct service intervals and the correct fluid added. If you do not replace it as suggested, you run the risk of ruining your transmission when you do replace it. I suspect this is the case. Overfilling does cause the transmission to overheat in some cases, but the fluid is usually pushed out the vent tube when this happens. I think at this point the only repair will be a rebuild. A qualified mechanic should be able to determine if this is in fact the case and suggest a good solution to fix your gears/transmission. This might involve removal and partial disassembly of the unit for inspection.