Oh dear, that doesn't look very unhappy!
Generally, it doesn't look like a disease. I notice that there does appear to be some relatively healthy growth towards the back, near the bottom. This suggests to me that it is very recoverable.
The first thing I would do is remove it from the current pot and either transplant it into the ground, or into a larger pot - at least 4 or 5 times bigger than the existing pot. Give it some fresh potting mixture (well draining) and a good water.
Keep the soil moist, but not saturated.
Also, I would remove all the dead growth - don't just pull of the leaves, actually remove by cutting the stem below the dead growth, leaving just what I believe will be healthy stem - new growth buds should emerge in a month or two.
After a month or two, I would hope you would see improvement.