A quick search turns up "Charcot–Wilbrand syndrome"[1]:
a patient loses the power to conjure up visual images or memories, and furthermore, ceases to dream during his sleeping hours
but it seems to be related to patients with some kind of brain damage.Personally, my ability to visualize is heavily affected by my habits, particularly sleep, diet and exercise. Have you had any lifestyle changes recently that might explain changes in your cognitive performance?
[0]: https://en.wikipedia.org/wiki/Aphantasia
[1]: https://en.wikipedia.org/wiki/Charcot%E2%80%93Wilbrand_syndr...