I am facing this problem

Hi abd_1989aa,

Since it's a rented server you've been banned from, you'll need to contact the owner to ask if they can unban you. However, it may be easier to play on a different server.

How do I contact him?

It differs between servers, but I can't really give you in-depth information, as I have no idea which server you were trying to join. However, you could start by checking the details for that server in the browser, as that might give you some information or show you who the owner or the admin is. Like I mentioned, though, it'd probably be easier to just find another server to play instead.