Hi Ursula,
It could be build up of hair, dirt and such blocking the drainage pipe. You can detatch it from the pipe in drains into and have a look, if you can detatch the pipe from the rear of the washing machine that would help too, you can flush water down one end and see if it flows out the other end appropriately.
If not then there maybe something blocking the drainage pump inside the washer or the pump itself is starting to fail.