The plant may be weak enough at this point that replacement is more practical, but we can provide some guidelines. We don't see any symptoms that suggest an exact reason for its decline, but with essentially no foliage left, it would be very difficult to diagnose at such an advanced state. Leaves that shed progressing from the oldest (lowest) first then up the stem suggest either a soil moisture issue or not enough light. Both too much and too little root moisture can cause similar symptoms, so are hard to tell apart. Since rubber plants are very drought-tolerant, soil that was too wet is more likely in that example. Plants that are exposed to ethylene (like from a natural gas leak or near lots of ripe fruit) can drop leaves in response to that gas because it can act like a plant hormone, but this isn't likely if you have any nearby plants that are not having problems.

Tropical figs/Ficus like rubber plant prefer to get somewhat dry between waterings and must have good drainage so the potting mix doesn't stay too wet. Make sure the pot has drain holes and if a saucer is used under the pot (or is attached to the pot), remove it and/or empty it promptly after watering so the collected water doesn't soak back into the soil and drown the roots. To determine when to water, you can feel the soil about a finger's depth down and wait until it gets relatively dry to the touch at that depth before watering well. (When watering, excess water should freely drip out of the bottom drain holes so you know the entire root ball was moistened.)

Light should be bright but indirect, which we realize is a somewhat poorly-defined term. They're pretty adaptable plants fortunately, but essentially should avoid too many hours of direct sun yet also should not be too far from a window, especially if that window faces north or east and doesn't receive much sunlight in summer. Placing the plant within about a foot or two of the glass of any but the sunniest windows should be fine. (Even then it's probably ok, especially once acclimated, since when grown in greenhouses at nurseries they're getting much brighter light than this, even when nurseries use layers of shadecloth over the crop.) If using grow lights because you don't have any (or enough) natural light, how far the lamp should be placed from the leaves depends on the intensity of the light it produces. Those that put out only blue-red/pinkish light are harder to judge for brightness but probably aren't too intense for a Ficus unless they are very powerful lamps and placed very close to the leaves. It's more often the case that artificial lights used are too dim or not placed close enough to the foliage (within a foot, more or less, of the top leaves is usually needed).

Potting mixes can vary considerably in how well they retain water and drain, but many can be used with success once you get used to how often they dry out so you know how to adjust your watering. Or, you can add ingredients like perlite to mixes that tend to stay wetter so their drainage improves. (This is one reason probing the soil with a finger is so useful, because you can't always tell visually how dry the soil around the roots is, and you don't want to rely on a set schedule since the plant's needs will change over time.)

Potting mix that is over a year or two old -- in the same pot and with the same plant growing in it -- may need to be replaced with fresh, as it can accumulate minerals from tap water and unused fertilizer residues over time, and this buildup can stress or damage roots. Old potting mix also slowly degrades as it ages and won't absorb or drain water the same way it used to (it might be harder to re-wet or might stay too wet for too long).

Ficus do well under-potted, meaning they are more at risk of root rot or other root problems in pots too large than the somewhat cramped conditions of pots on the smaller side. This is mainly because rootbound plants are not as easily overwatered. When increasing pot size as a plant runs out of root room, it's generally recommended to go up only one pot size. For example, move a plant from a 4-inch pot to a 5-inch pot, or from an 8-inch pot to a 10-inch pot. (The difference will only be one or two inches as you go up in size.) Just like with outdoor plants being transplanted, loosen the roots if any are tangled or matted together prior to putting it in the new pot and filling-in with new mix. Lastly, avoid using outdoor soil types (topsoil, pure compost, or other "garden" soils) when potting indoor plants because these soil types tend to hold too much moisture for too long. Potting mix (also called potting soil even though they're not true soils) is formulated so it drains better.

Hopefully this information helps you with making an educated guess as to what stressed the plant, and can help you succeed with another (or this one, if it pulls through).

Miri