A vein occlusion to the eye is much like a stroke (to the brain) or a heart attack. Once the damage is done, there is nothing to be done to reverse the cell deaths.
However, sometimes a vein occlusion causes decreased vision because it also causes leakage of fluid into the macula (center of retina) also called macular edema. This can be treated with laser or injection of medications. After 5 years, this is probably not the case.
It's also important to find the cause of the vein occlusion (diabetes, hypertension, cholesterol, glaucoma, etc.) to prevent future episodes.
I hope this helps and thank you.