This is a new thing that Yahoo has implemented, and you will have to disable AdBlock Plus in order to check your email. I'm not certain whether Yahoo detects all adblockers or just AdBlock Plus though, so you might want to give something like UBlock a try and see if that will still allow you to check your email.
I understand where Yahoo is coming from; they are providing a "free" service that is paid for by advertising revenue. If everyone is blocking ads, there goes that revenue. I personally stopped using Yahoo mail years ago, but if you want to continue using it, you might want to white list it on your adblocker. Maybe you could forward it to your gmail or other email account as an option.