I'm sorry to hear your pond is in such a mess. I used to have the same
problem until I kept the pump running over winter. A pond hoover would
clean up the bottom and you could try replacing a third of the water
initially. I wonder if you have frogs in the pond because I have found
the water tends to get dirtier when the tadpoles emerge but clears up
later. Once you have the pump running again, and hopefully you have a
UVC, the pond should start to look a lot clearer.