Did you have a sharp winter?

No it was mild and now it's unseasonably warm. I'm in California.

Should I use some kind of tree-patching thing on the crack?

One think I can see - in addition to the amazing splitting of the trunk in half - is that the root crown is buried. Is that a recent development?

Yes, actually. I was digging up vole tunnels and probably buried it in the process. Can that cause this problem?

If you’re in a warm zone in California and you have no leafing out or suckering from the ground it’s dead. I wonder if you have gophers and not voles gophers are a big problem in parts of California. They will eat the tree roots.

The tree is in a gopher cage. I have both voles and gophers, but the backyard has voles. If it's dead, what caused it to split like that? When you break a branch, it seems alive.

Should I patch the hole?

Why patch it? Something is terribly wrong with the tree for not being leafed out or suckering by now. It’s either dead or on its way there. figs grow so quick in California and are easy to acquire. Panache is a vigorous variety with variegated fruit, excellent taste, and easy to find. Makes good rootstock if you want to graft multiple varieties on it. I would be starting over if I was you.