First, you need to slow down and take a deep breath. Charging head-long into a problem like this will result in frustration in data loss. You have two issues to address... your boot device is failing, and you've got a drive missing from your pool.
Do you have a configuration backup? I'm going to guess no. First, I'd start by replacing the boot device and installing the latest production version of FN. If you're using a USB stick for your boot device, STRONGLY consider moving to a single, small SSD (I've literally bough a handful of $20 40GB Intel 320s off eBay which are perfect for this task). Get your new FN up and running. Then, import your pool using the GUI. Once that's happy (check zpool status), then you can start determining why that one drive went offline.
Just to be clear, your data isn't tied to the FreeNAS install. You can destroy your boot device and restore from a configuration backup, or import a pool into a bone-stock FN install with zero data loss.
Do you have a configuration backup? I'm going to guess no. First, I'd start by replacing the boot device and installing the latest production version of FN. If you're using a USB stick for your boot device, STRONGLY consider moving to a single, small SSD (I've literally bough a handful of $20 40GB Intel 320s off eBay which are perfect for this task). Get your new FN up and running. Then, import your pool using the GUI. Once that's happy (check zpool status), then you can start determining why that one drive went offline.
Just to be clear, your data isn't tied to the FreeNAS install. You can destroy your boot device and restore from a configuration backup, or import a pool into a bone-stock FN install with zero data loss.