Diagnosing dead Pixhawk?

We have a BlueROV2 from 2017, with a Pixhawk 3DR. I recently installed blueos 1.0.1 on the Raspberry Pi, and updated the Pixhawk to Ardusub 4.1.0, after not having used it for a couple of years. We had the system running and were able to dive, but were getting frequent “EKF3 IMU1 forced reset” messages, pre-arm check failures and warnings about yaw inconsistent, even after recalibrating all the sensors. We then lost the connection. On power up, we can see video in QGroundControl, and we can ping, but there is no heartbeat in blueos.local, and the Firmware link says that no board is running. On the Pixhawk, the FMU lights are:
PWR: flashing green
B/E: off

and the IO lights are:
PWR: solid green
B/E: solid orange
ACT: flashing blue

Something (the Pixhawk?) is beeping every few seconds, one long tone and two shorter ones, rising in pitch.

I tried the restart autopilot button in blueos.local, but it doesn’t seem to change anything. I can’t see anything disconnected between the Pi and the Pixhawk, although I will open up the electronics tube to check. Is the Pixhawk dead, or are there more things I should check? And if it is dead, should I just buy a Navigator instead?