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It would seem that your grafted citrus has failed. What I see growing, now, is a commonly used rootstock. It is given the name, "Flying Dragon" and does not produce tasty fruit. They will be quite sour and bitter.

This can happen if the top dies, or gets pruned back below the graft point. Once the scion (the grafted top) is lost, it cannot be recovered.

Usually, in container, this can be caused by infection. This appears bacterial. It can happen from remaining too moist for too long, or not having drainage to allow extra moisture out of the container.

Though this will not produce a sweet, or even palatable, fruit it still can be used. If you want to keep it, you will need to treat with a fungicide, first. This article will help:

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

This article will help you to understand why trees revert back to rootstock:

https://www.gardeningknowhow.com/ornamental/trees/tgen/can-grafted-trees-revert-to-their-root-stock.htm

Here is an article to give you more information on "Flying Dragon" rootstock:

https://www.gardeningknowhow.com/edible/fruits/trifoliate-orange-tree/trifoliate-dragon-orange-tree.htm

Sap will leak in the case of fungal or bacterial disease, usually bacterial, and insects problems.

Because of how the damage appears, I would say that this is likely bacterial. A single use of fungicide will not be enough to kill off the infection, so multiple treatments will be necessary. Depending on the infection, it may be very difficult to rid.

To prevent future infection, once you do manage to kill off the existing infection, will require keeping the soil from staying moist for too long. With that style of container, rain will build up in the drain pan. You will need to make sure to dump off all extra water that ever accumulates in the attached drip pan, or simply put it into a container with plenty of drain holes to allow water to leave freely.

Make sure that the soil dries down to at least 3 or 4 inches between watering.

Here is an article that will help you to care for a lemon in container:

https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm

It can take months for lemons to ripen. I would leave them on the tree, they could still ripen inside. The tree will need a lot of light and some humidity indoors.

https://www.gardeningknowhow.com/edible/fruits/lemons/lemons-stay-green.htm

https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm

Here are two articles with tips about lemon tree problems.
https://www.gardeningknowhow.com/edible/fruits/lemons/common-lemon-tree-diseases.htm
https://www.gardeningknowhow.com/edible/fruits/citrus/curling-citrus-leaves.htm

If you're able to eliminate all the possibilities suggested in these articles, I would say it's likely that there's a watering problem. This article explains the proper way to water your tree: https://www.gardeningknowhow.com/edible/fruits/citrus/tip-on-water-requirements-for-citrus-trees.htm

There is no reason to do so, at this time, unless the idea is to keep it shorter. Then, you would prune the top, though.

Here are some articles that will help:

https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm

https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-pruning.htm

Unfortunately, your photos did not come through. I have even less information than you have!

It could be a multitude of issues, and likely, is more than one of them. If the light, humidity, temperatures, or soil moisture changes rapidly, or is no longer in a state that promotes the growth of citrus, it will shed all of its leaves and fruit.

Unfortunately, there are a lot of factors to pick through. I would start by using a horticultural grade bulb 100 watts or greater. Do not use cheap LED panels, as they will not suffice for citrus.

https://www.gardeningknowhow.com/special/greenhouses/grow-light-terminology.htm

Make sure to feed with citrus fertilizers, and plant into a citrus soil to take the guesswork out of preparing for the picky trees.

https://www.gardeningknowhow.com/edible/fruits/lemons/learn-about-fertilizer-for-a-lemon-tree.htm

These articles will help you to grow the tree:

https://www.gardeningknowhow.com/edible/fruits/citrus/growing-citrus-trees-indoors.htm

https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm