I had a Netgate 3100 go down after a storm recently and I had to do a reinstall of the software. Below is the process I went through.
1. Create USB install with pfSense-plus-SG-3100-recover-21.02-RELEASE-p1-armv7.img.gz image. I used etcher to create the USB stick.
2. Insert USB and boot the 3100.
3. Stop the boot to where you have the Marvell prompt.
4. Type "run recovery". (You may have to type "usb reset" first if it does not recognize the USB drive).
5. Walk through install.
6. Reboot and take out USB.