The serial connection between the drone and the camera is disconnected, most probably the PWM/Tx (the white wire between contact plate and slip ring) line or both.
Do a visual check of the white and the green wiring.
The white wire and green wire must both have continuity. Ultimately all the way between the drone main board connection and the gimbal board connection. But there are a couple of common places that problems occur:
1.) Stuck contact springs on the mount plate. If any of the contact springs are lower than the others, pop it back out to the normal position.
2.) Broken wire between the top of the slip ring and the camera side contact plate. It is obvious if wire is in two pieces. But the insulation is very flexible. The wire inside can be broken but the break is hidden because the insulation stretched instead of breaking. If there has ever been an event where the dampers pulled out, a broken slip ring wire (visible or not) becomes a higher possibility.
3.) Bad slip ring. This one is normally easy to find by checking the continuity of each wire going in and coming out. There are two pairs of wires that are soldered to the same pad contacts. Those two are not likely to be the issue in your camera, but they should be checked anyway. Ensure they have continuity between them on the gimbal board end. (Again, the issue is likely either the green or white wire). Also be aware the slip ring rotates. It can check good in one position, but fail in another.
4.) Pin(s) not fully inserted in the connector on the gimbal board end. If there is ANY amount of the metal pin exposed on the back side of the connector, push the pin back in and make sure it engages the little plastic latch tap for it on the connector.
These are only the most common. There are more connectors, two connector boards, more opportunities for broken/ crushed wires, slipped pins, bad solder joints and general mayhem. They key is the full continuity check between the main ESC board on the drone and the gimbal board must be good.