I had the same issue. After a few days without connection from my ISP my CyberGhost client stoped working.
It didn't want to open the client, but task manager did find it in the background, but I wasnt able to access it.
Tried to uninstal, reinstall and a bunch of other things, but nothing worked.
I finaly found this:
(557) VPN Not Working in Windows 11/10 – Fix VPN Problems [Tutorial] - YouTube
After completing the first step in the tutorial and restarting the computer, it worked.
Hope this helps