My icons dispeared

Hi, I am trying to create web page for an Association for efficient tools of waste collection Zenzo – I am not a web builder/developer, my boss just thinks I am the most technically type. ??
I have one problem with our page. All the icons disappeared. I don’t know what happened, they are just gone (search icon, icon for closing the main menu… fas fa-long-arrow-alt-up) In HTML code I see it, but I cant see it on web page. Can anyone please help me with this?
Thanks a lot.

The page I need help with: [log in to see the link]