I only got one very small lemon on it and it stayed green. I have it in a pot.
There are many reasons for this to happen. If you do not fertilize it every few months then this can be the issue. I would also recommend having dolomitic lime, and iron sulfate on hand. You will want to apply these at least once per year.
The next possible issue will be not enough light. If this is indoors, then you will need supplemental light to produce fruit. If this is indoors, it would be wise to have at least 200 watts of horticultural grade lighting to produce fruit properly.
Temperature can also play a factor here. If it doesn't remain quite warm, then this can halt fruiting.
Leaves are getting brittle and turning under. Don't know how old the tree is, acquired it after a couple of days of the weather being pretty chilly. We haven't had first frost yet, but Claressa to it. It is a potted tree and has about 6 lemons on it. Have never cared for a citrus plant below, so I guess I need beginner's lessons.
"Leaves getting brittle and turning under" This may indicate water deficit/drought stress from under watering. Or transplant shock from root disturbance if you re-potted it or planted in the ground.
https://www.gardeningknowhow.com/edible/fruits/citrus/tip-on-water-requirements-for-citrus-trees.htm
Large thick skinned fruit. Really bad shape. Is it possible to hard prune to bring it back in? From Sydney Australia
You can, but it will not fix your fruit problem. That is a separate issue that needs to be addressed.
Here are some articles that will help:
https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-fruit-thick-rind.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-pruning.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm
I planted a 4 ft Eureka Lemon Tree two months ago. At the time it had several leaves and looked healthy, but the leaves have gradually fallen off and the tree is now leafless. It gets the appropriate amount of water, I believe, and plenty of daily sunshine. I have scratched the vine from the top down to the root ball and it is all green, so it appears to be alive. I have also pruned the branches, in hopes of some growth, but it just sits lifeless. Any suggestions on what the problem could be?
I suspect it's part transplant shock and part soil related. Is that soil sandy? If so, it needs some compost added.
You might want to take a soil sample and have it analyzed as well. Your local extension group can do that.
Purchased at Lowe's about 3 years ago. This is the first bloom and first fruit from this tree.
Unfortunately, your photos did not come through. I am unable to see the damage. Without much context, I will not be able to help very much, unfortunately.
Eureka's have long been known to have a tendency to mutate. This is where my favorite citrus in my garden comes from... The Variegated Pink Lemon!