I used to play VRChat all the time with the ‘Known User’ rank, then I stopped for like a couple months (maybe even a year) and I got back into it it and, I somehow jumped down to ‘Visitor’, is this normal? How can I get my levels back up again? Do I have to do it all over again and play for a year straight?
Thats not how trust rank works.
Maybe your account was compromised, and you were moved to nuisance rank, or you have logged into like a steam account instead of your vrchat one.
If everyone unfriended them, the trust level would go down. But two levels seems a bit odd. I do have one friend who went from Trusted down to Known and they were at known for six months, but they are fairly rarely on.
Oh that’s probably what happened 
since all my old friends are gone.
It was probably from New user to Visitor in that case—I don’t exactly remember