pull the toilet. Make sure the trap is clear on the toilet, and then snake the toilet line.
A closet auger! ( Closet is in reference to a water closet or toilet) It can be purchased at most home improvement stores.
I do not enjoy plunging into cold water.
Sound like you have a clogged toilet and or a clog in your main drain under your toilet. Shut off water supply to toilet remove water from toilet tank. Puchase a wax ring for your toilet prior to disconnecting toilet from supply line and floor bolt on the right and left of your toilet. Remove toilet from floor if water is standing in pipe under where toilet was this is where your problem lies. If no water or clog present, turn toilet on side and lokk in hole at the bottomof toilet. Clear was is stuck in toilet clean off old wax ring install new one and put toilet back together.
Unless it is a very strange arrangement, turning off the toilet on the first floor should only shut off the water to that toilet. It should have no effect on the upstairs shower.
If it's leaking between the toilet and the floor, you put the wax ring in wrong. If it's leaking between the toilet and the tank, you need to replace the gasket.
The special bolts that hold a toilet to the floor.
All you do is lick the floor or you can drink toilet water.
If it is coming from the tank: NO. If it coming from the bowl: YES!
Call a plumber
When a toilet rocks like a rocking chair, leaks water around the floor connection or displays other signs of insecure foundation, the toilet flange needs replaced or reset. A toilet flange is the metal plate located below the toilet which connects the toilet with the sewer outlet pipe while securing the toilet with the floor. When a toilet is initially constructed it is secured to the floor with screws and the junction is sealed with a liberal application of beeswax. When a toilet is securely fastened and maintained properly, proper function including no water leakage or odor present is the norm and not the exception. An indication of possible need of attention is the presence of black mold on the floor near the base of the toilet. Also evidence of water leakage or an unpleasant odor originating from the toilet base is a reliable indicator of the need for a toilet flange repair. Regardless of who does the repair of a toilet flange, the following criterion is paramount for a job well done. It is also important to follow certain procedures, from diagnosis to completion of the job. Tools and materials: screws, screw driver, flange bolts, wax rings, caulk and a flange repair kit. Remove the Toilet: After shutting the water supply to the toilet, flush the toilet water from the tank and bowl before releasing the toilet from its floor connection. Unscrew the bolts and nuts on the sides of the toilet base. Secure the bowl and tank and lift the toilet from the floor connection. Repair the Toilet Flange: Old wax must be cleaned off from the old flange. Flange bolts may need replaced due to corrosion from water leakage. Position the flange bolts in the holes of the repair kit and place it on the floor as the previous flange setup. This step ensures proper placement of the flange and prevent leakage. Fix the Flange and Toilet: Follow manual instructions of the original toilet manufacturer when screwing the flange to the floor. Place wax rings and remount the toilet with bolts and nuts.
Many 'slow' water leaks will waste several gallons of water a day. Also, the floor around the toilet can be damaged if the leak is dripping onto the floor. Toilet repair kits are very cheap, and easy to install. Check at your local hardware store, Home Depot, or Lowe's.