I may have had a similar problem. Both of my Hikvision cameras had their PoE feature stop working due to weather exposure. In both cases, I fixed it by buying cheap *passive* splitter/adapters for each end of the Ethernet cable, and injecting DC power into them using leftover transformers. The only trick was choosing transformers that produced the right DC voltage, taking the long cable run and voltage drop into account. It was hit-and-miss, but fortunately I found a voltage that worked in both cases.
As I understand it, the right way to do it is to purchase *active* splitter/adapters, not passive ones, which have the smarts to adjust the DC voltage to the camera to the necessary level on their own. Then you can continue to use your PoE switch (or whatever is injecting DC voltage into your cable now), without having to manually tweak the voltage yourself.