after upgrade to windows 10

Recently I upgraded my computer to Win 10 (I had Win 8 originally). After that I only have Windows Defender. Norton Antivirus has disappeared from Apps list.

I thought of reinstalling the application. I got an icon on desktop and one in "Start" menu. However Norton Antivirus doesn't launch at startup. A double-click on any icon doesn't launch the application neither. Though I had previously (in Win8) activated my Antivirus program I expected a new activation request after a reinstall. I didn't got to that step neither.

Disabling Windows Defender doesn't help.

What should I do now?