You cannot answer 'Don't know' to this question as we won't be able to help if you don't know where your internal stop tap is or if it's working.
Please note: You may need to check if you have more than one stop tap as sometimes additional stop taps are concealed behind units/cupboards.
Checking your stop tap
You should check that your stop tap is working properly and will open fully.
To fully open the tap, you should gently open and close it several times until the number of turns from open to closed is constant.
Do not leave the tap fully open. Turn it back a quarter of a turn to stop it seizing up.
If you find you cannot turn the stop tap i.e. it has seized up, you may want to call a plumber to replace it for you before continuing with this test. Unfortunately, we will not be able to assist you with repairing or replacing your internal stop tap and your stop tap usually needs to be working properly before we can investigate further.