Remove the drain cover/grating. If the water is sitting high in the gully, use a bucket to bail some of it out. This makes it easier to see what is causing the obstruction.
If you have tried rodding and jetting the drain without success, or if the blockage is located underground between inspection chambers, you may need a professional drainage engineer. They use high-pressure water jetting trucks and CCTV cameras to locate and pulverize stubborn blockages like tree roots. unblock drain pipe outside
If surface clearing doesn't work, use physical tools to reach the obstruction: Remove the drain cover/grating