The volume icon in Windows 10 turns gray and cannot be selected. What to do?

Microsoft Community Solutions:
1. Left click on windows icon and choose settings.
2. Click on display tab.
3 Change the size of text to 125% Then apply.
4. DO NOT LOG OUT YET. Choose sign out later!
5. Change the text size back to 100% and click apply.
6. Now sign out and sign back in.
Your volume icon will be back. =)
Simple operation is to right click on the desktop, select display settings, up or down the current font display size, select logout (this time generally logout The back icon will appear).
Or log out later, change the current font display size again, then apply, logout, volume icon appears.