-Hold press the arm/disarm safety switch while plugging in the Pixhawk. Keep holding the switch for a few seconds.
-Open QGroundControl or other uploading based method.
-Flash your firmware. Wait for Pixhawk reboot. Connect to Pixhawk.
-Through QGroundControl, under "System" tab, set all parameters to default. Save. Let Pixhawk reboot if necessary. -Close QGroundControl. Unplug Pixhawk. Plug back in. Let Pixhawk initialize first.
-Open QGroundControl, Set up your preferred vehicle type. Let Pixhawk Reboot
-After it reconnects, go to the radio tab. Pair your receiver and transmitter if necessary. You should be able to receive RC commands now.
***The main fix here is holding the safety switch button down while powering on the Pixhawk. This throws the Pixhawk into bootloader mode. You aren't necessarily flashing a new bootloader but it has something to do with syncing of firmware versions. Don't hold me to that. That is my guess. This was the only method that worked to get RC control back. I tried this fix across multiple different Pixhawks with stable/beta/dev/custom_default/custom_lpe firmware.
Usually you are supposed to just format the microSD card to fix these issues but that didn't work for this specific error. Although to be on the safe side, a good Format to FAT32 couldn't hurt.
i hope this works for you.