Mac w/ VPN: You do not have permission to access...on this server

I use Duck Duck Go as my browser w/ 1OS 10.13.6 (high Sierra). There are sites I have been able to sign into for years that I suddenly am denied access to. I don't understand why, nor do I know how to fix the problem. Any guidance would be greatly appreciated, but please write to me as if I were in the second grade. Simple! Thanks.