I received a warning from the city for not mowing my NEIGHBOR'S LAWN?!!!!?

I am renting a house with a huge lot next door that belongs to someone else. I used to mow it but it took me hours and it was so hard cuz it has huge logs and boulders and is pretty bumpy. So I quit mowing it and just mowed my front and back yards. I just received a warning from the city that I need to mow it or I will be sent to court. Is this right? What can I do about this???