Please can anyone help?

My downstairs loo is not flushing properly. When flushed the water
level fills really high then drains lower than the level of the pipe.
One of my outside drains is also high with water. We do not have an
inspection cover on our property and the old lady next door down just
had hers covered and a small 6 in cover put on.

Any ideas gratefully received,
Thanks
Taffy