Problem recording in stereo

I’m pretty new, but was sucessfully transfering cassettes to wav files, but somehow now Audacity is only recording on one track. The screen shows 2 tracks (stereo) but only the bottom (R) shows any wave. The top (L) shows only a straight line.
The recording level meter is working, and it too shows only the lower R track as recording. I haven’t done anything that I’m aware of to cause the recordings to change to mono. I’ve checked the audio I/O tab and it is set to “2(stereo)” as it should be. I’ve plugged and unplugged the connection cable into every possible jack without success.

I’m using a karaoke machine to play the cassettes, it has RCA red/white audio output jacks on the back. I bought a cable with the red/white connectors on one end and a minijack plug on the other. There’s a row of four little different colored line in ports on the back of my computer. (I guess that’s what they are, my computer manual does not identiy them) The only other thing plugged into one of these ports is my computer speakers. Only one of the other ports works for Audacity. Like I said, it was working fine-showed 2 tracks being recorded, then all of a sudden, the upper track shows a straight line.

I don’t know what else to check. Could it possibly be a short in the cable? Any suggestions appreciated. Thank you very much!!!

If it was working, with that same set up, and now it isn’t, then it could easily be a duff lead.
(note also that the microphone input is mono, so you can’t get stereo through that particular socket, but you should get stereo through the “line” socket ).