Welcome to the forum (finally)!
Well, they aren't completely invisible. But it looks like you have a theme or maybe desktop environment or GTK option which is substituting those light gray icons for the usual.
I'd better pay close attention to the answer here, since I'll be switching to Linux soon! Aside from trying a different environment, or maybe there's a theme choice, or other display choice you can make, I'm not sure how to fix it. But we definitely have other Linux Mint users who can help.