It should be a matter of pulling down the audio input selector (microphone symbol) and picking the same thing you used to pick before – typically Stereo-Mix, or WAV-Out. Something like that.
Older versions of Audacity would occasionally configure this tool wrong and it wouldn’t work, but the later versions work very well.
You may need to go into the Windows Control Panels to make sure Stereo-Mix is still available – and select it there to make sure. If it doesn’t appear there, then there’s something wrong and Audacity will never find it. Windows has to be configured first.