Your schematic does not show any of the supply components so is 'incomplete'.
There are 2 ways to implement a 'protection diode' with differing results.
One is to put in SERIES with the supply so if you connect it to DC reversed it will simply not work. This would also be OK if you actually stuck AC into it and it WOULD work but probably hum quite a bit.
This would NOT trash a 1N4002 diode.
The second is to have it REVERSE connected ACROSS the supply so that if you connect a supply backwards the diode conducts as hard as it can and, given a reasonably limited current capability will prevent the reverse voltage being more than about 0.9 Volts which is very unlikely to damage anything. THIS method relies on a series resistor or fuse to blow hopefully before the diode catches fire. This method is often used in small battery gear as it does not 'waste' 0.7 Volts during normal operation.
There is sufficient resistance in series with the transistors to protect them so the more likely part to have failed is the op amp.