I had the same problem two days ago and I solved it using this method, which I kindly ask you to try out:
In Windows Search Bar, look for "Turn Windows features on or off". Launch it and scroll down until you find "Windows Subsystem for Linux". You'll find it not checked, so you need to check it. Then, your PC will ask your for a reboot. After this, try to launch Ubuntu again and it should work just fine.