There is a leak...
Try to locate the cause of the problem.
Is is rain water?
Is it the flooding showerof the neighbours above or a washing-machine?
Is it a blocked toilet?
Is it a blocked gutter or a flat roof that has its waste-pipe blocked by leaves or any other object (a ball, a bottle, plants, etc etc.)
In many cases you can immediately stop the leaking by closing the water mains or a tap nearby or unblocking a waste-pipe.

If you cannot find the cause or there is damage, please call the landlord as soon as possible.