Help needed asap!!!?

Question

i turned on a tap in my rented house, and now it wont turn off. its for one of the showers and its quite old! the water pressure is not great but its still wasting alot of water. we cant call the landlady as one of the windows is broken and we want to get it fixed first, and we cant affort to call a plumber! help? it needs to go in as its turned, to turn it off but the mechanisim that used to do that is worn. help? please!