Polarity mismatch or partial short. Fix: Swap the positive and negative wires on that channel. If volume returns, the original wiring was reversed.
Using the wrong wiring diagram can lead to: Toyota 08600 Radio Wiring Diagram
Always disconnect the negative battery terminal before touching any wires to avoid short circuits. Are you planning to install an aftermarket Android head unit , or are you troubleshooting an original factory radio that has lost power? Wire Almost Any Basic Toyota Stereo Polarity mismatch or partial short