Barbara,
Unfortunately, I have had the same experience occur to me, a number of times during the last year. By jiggling the cable, just below the point where it enters the headphones, I could intermittently re-obtain stereophonic sound. I also found, after some research, that if I eased the plug out of the headphone socket, by about a couple of paper thickness's, the stereophonic sound came back, but again, fleetingly. It is all rather arcane, and I thought that the next stage would involve something nasty with frozen chickens.
The trouble is that there has been a decrease in quality maintenance as a consequence of the proliferation of cheaper headphones on the market. Everyone these days needs to listen to their cat videos or to gossip with colleagues over zoom, hence the proliferation of headsets. With profit margins getting slimmer the first thing to suffer is quality maintenance. If it works heading out of the factory, great, but you are not building a piece of equipment that will last for years, because otherwise, where is your return custom and your profit.
Back in the 80's, when I first got hooked on the electronic production of music, everything cost a fortune, but equipment was built to last. In a niche market, if you antagonise your client base with poor quality, expensive rubbish you very quickly run out of customers. The Roland headphones I bought in the early 80's still work , though I must remember to buy some more foam ear pads to replace the perishing ones.
With a new set of headphones, the soldered connection between the cable and the headset is usually the weak point and one or both connections will fail with constant changing strain on the cable. One used to be able to mend these things, back in the day, but with moulded plastic headsets you can't get inside them to re-solder the loose connection. With the poor connection to the headphone socket, this is a consequence of the low manufacturing tolerance on cheaper jacks; they go into the socket but they don't quite align with the contacts inside the socket. Decent quality jacks are expensive for reasons other than the gold plating.
There were two solutions suggested on the Web. To buy a set of headphones which allow the cable to be plugged in via a jack. This means that any strain on the cable will pull the jack loose from the headset, rather than damaging the internal soldering. The other was to replace the standard headphone jack, at the other end of the cable, with a decent quality one from an electronics store.
My "solution" was to buy a reasonable quality set "for best", which only gets used when i really want to hear what I'm playing around with. I also have a sturdier, cheaper headset and microphone "for gardening", which I use for everyday mucking around, and has a jack and socket inserted into the cable close to the headset. It is not perfect but it will alleviate any major strain on cable by pulling apart.
I would also have a rummage in all of your audio settings to ensure that you haven't inadvertently converted your headphones to mono or single channel, but otherwise, we live in a throwaway society, and all one can do is to buy responsibly once one has identified the problem.
Welcome to Pianoteq, Barbara, I'm sure that you will have as much enjoyment with this application as everyone else has had, and also loads of delighted "Oh I didn't know it could that!" moments.
Michael
Pianoteq 8 Studio plus all Instrument packs; Organteq 2; Debian; Reaper; Carla