Cargo ships abandoned my city. How do I get them back?

I had exactly the same issue and it was caused by a broken road connection to the harbour that was impossible to see without removing the harbour. Vehicles passed the harbour as expected but it was impossible for them to enter the harbour because the road was laying above the piece of roed that belongs to the harbour. But that was invisible. During construction I used traffic++ and overlapped the roads in order to create a nice industrial harbour sea wall. I made some refining work after the harbour initially worked. This I assume brought up the issue that the road connection of the harbour asset was not realized correctly. After removing and re-building the harbour it worked again. I struggeled with this for days as well, looking for obstacles in the water, increased the water depth, turned the harbout off and on again... before I decided to break it down and start from scratch.