no sound from Windows and it’s apps until I launch Cubase 12

Bizzare, Today I get no sound from Windows and it’s apps until I launch Cubase 12. I use a UR824 interface. Equally when I exit cubase the sound of say YouTube stops. Yesterday I was learning to use Kontakt within cubase. After that I closed cubase and launched the UR824 DSP mixer app and engaged it’s loopback function to record sound from YouTube but there was no sound from YouTube. I’ve had no problem doing this in the past. I’ve now cleared the loopback check. Today I still have the problem ie. no sound fromWindows unless cubase is open. Can anyone explain? This is driving me nuts! Many thanks Simon

Cubase 12 pro

Windows 10 PC Intel(R) Core™ i7-6700 CPU @ 3.40GHz 3.40 GHz 16GB ram

Yamaha UR824 interface.

Nektar Impact midi controller

Komplete 14 Standard