A blocked pipe. The water can not go down the pipe, so it comes out in your sink.
Drain line it's connected to is clogged.
There is a main drain that runs from the basement through the roof where it is the vent for the system. If you look at the layout of the house, the upstairs bath is most likely over the downstairs one or the kitchen. If you look on the roof, you will see a 4 inch pipe coming through the roof that should be over the upstairs bath. The sink and shower drain into that and then to the basement and out of the house. Most likely the main stack is in the wall behind the toilet.
Do you have an upstairs tenant or the sewer is backing up.
Check the upstairs taps for blockage in aerator or internally.
because the upstairs toilet drain was clogged up.
Is the water not hot? Or no water coming out the hot tap? not comming out at all
Have a plumber snake out your drains.ANS 2 - Install a flapper valve section on the sink drain pipe so water can go down but not up - cheap and very easy to do.
Open the radiator drain and let drain into a catch basin. Run water through the radiator by filling through the radiator cap with a hose till the water coming out is clear. Fill through the overflow bottle if there is not a radiator cap present.
The main drain is plugged. Not completely, but enough that it is easier for the upstairs water to come out in the shower and floor drain instead of going out the drain.
You can get a back water valve. It is similar to a check vale/ the back water valve goes on your main sewer line out side of home. It is desined to stop black water form coming into your home if main line stops up. Thay do make a device that can be installed into an exsisting drain it lets water through your drain but will not let water back up through you shower drain.
Check the AC condensate drain, it's probably plugged.
you will need to drain the storage tank == First, you turn the water off at the main, then turn on all the faucets and let them drain until there is not any water coming out. The absence of water will prevent their freezing.
the drain hose is plugged