Give it a severe pruning and that will force it back to production. Almost every fruit tree will stop producing as it gets old, but by pruning it, you're tricking the tree into thinking it's still young. If its been years, it will need a almost complete cutback, but be careful, just a little too much taken off will kill your tree.
I've got the opposite. A mutant tree with finger-length thorns that grows lemons as big as my head. Literally. There's a normal amount of lemon fruit in it, but the skin is, like, 2 inches thick!
What you have growing is a citron tree. The fruit are supposed to look like that and are grown for the rind. It's actually an ancestor of the modern lemon, not it's mutant child.
I've had a lemon tree in my backyard. Nothing has grown for he past 3 years.