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.
Understanding the "why" can help you choose the right fix. Common culprits include: The 5 Most Common Signs Of Blocked Drains | Checkatrade unblock drain pipe outside
Before touching the drain, put on your gloves. Outside drains can harbor harmful bacteria. If you have a septic tank system, avoid using strong chemical cleaners as they can kill the beneficial bacteria in your tank. Remove the drain cover/grating