Hydro Plant not allowing water flow through
Since we figured it out in the comments here the answer:
Your dam is placed to close to the water source so your river has stopped flowing.
Apparently the the system works this way: There is a source point which generates water until it reaches a certain level. If that level is reached it stops producing water. So when you build your dam your water level has risen above that threshold and there is no more water being produced, hence you created a new lake. Congratulations on that. ;)
Move your dam further along the river to fix the issue.