I tried to use the usual code:
response = requests.get (url)
response.json () OR json.loads (response.text)
BUT
it returns JSONDecodeError Expecting value: line 1 column 1 (char 0) — no matter, which website I am trying to parse, with headers or without them. 
However, response.status_code = 200 and response.text returns the whole html-content. Two days ago everything worked completely fine, and now this disaster happens. I asked a couple of friends to try the same — and everyone failed to succeed, everyone gets this error — using Jupyter Notebook (+Anaconda, Python 3.7), PyCharm (+Python 3.9) and Google Colab.
Please help :(

JSON is not HTML. If response.text contains HTML, it is expected that json.loads() cannot parse it.

> Two days ago everything worked completely fine, 
> and now this disaster happens.

It's unlikely that Python itself changed over those two days. It's more likely that the data source has changed.

Try this in your notebook:

    import requests
    r = requests.get('https://api.github.com/users/gvanrossum')
    print(r.status_code)
    print(r.headers['Content-Type'])
    print(r.json())

If that works, try it with your url.  If the content-type is not 'application/json; charset=utf-8', then you've identified a problem with the data source rather than with the tooling.

Thank you so much for the answers!!
The problem was indeed with the data source, they have fixed it.
Sorry for bothering you! 
Wish you all the best!!!