if you have a pump (in the pond), there'll surely be a cord going to it. Trace this cord and find where the pump is. Not a great idea to pull it up by the cord, but seeing your pond, you should be okay if you have to. Otherwise, root around and find the pump, pull it up, inspect for blockage and clean. I'd disconnect the hose and run it without, underwater, to see if it works properly. If so, then go to your filter and thoroughly clean it out. There should be the other end of the hose near/in/at your filter box. Disconnect that, reconnect the hose to the pump, run the pump and confirm the pump outputs water through the open hose. If all is good, reconnect the filter and you should be good to go.
If you have a lot of debris at your pump, you probably need to do a slow netting of the bottom to get rid of any larger debris. I'd also suggest, after doing this, to set your pump off the bottom for future use so you don't clog your system as easily. Filter boxes need regular maintenance and it may be your pond system needs this or more. I'd check every day after cleaning and checking, to see how quickly it's clogging. The smaller the filter collection area, the faster it clogs and the sooner you have to clean it.
An alternative is bog filtration; if interested, search the forums here for lots of info.
Hope this helps.