Visual imagery uses exactly the same brain areas as proper vision. (This was the original topic for my PhD so I have read extensively on this topic!)
The information from the eyes goes via the optic nerves to the lateral geniculate nucleus of the thalamus, deep in the brain, and then onto the visual cortex (also known as the occipital cortex) which is right at the back of the brain. From there, the information feeds forward to lots of parts of the brain, initially through the temporal and parietal lobes. Visual information relating to objects (e.g. colour and shape) is processed in the occipital and temporal lobes. Information relating to actions (e.g. how far away something that you want to reach for is) is processed in the occipital and parietal lobes.
If you are not having any difficulty with reaching and grasping or other visually guided actions then I would guess that you have something going wrong in the occipital-temporal pathway (also known as the ventral pathway of vision): an MRI might show a new lesion along that route, I would guess in the early part of the temporal lobe.
The fact that you have lost your ability to do visual imagery is actually fascinating for anyone studying visual imagery! I wish I was still researching - I'd have you in the scanner in a shot!!!
Unfortunately, I can't think of any treatment for it - sorry. Hopefully time will do the trick - practice might be a good idea too - teaching the brain how to do it again. It can work!