my website has been Server is under maintenance…why
That is a question for your host.
how it solved
As it is a question for your host, by contacting your host.
how i contact him
Ehm, by sending them an email or opening a support ticket
I am really not sure about your question. You need to know who your host is and how to contact them. That really isnt something anybody here could help with.
i changes my name server to cloudfare .can my hosting provider also change
Whats the domain?
Yes, that is a message directly from your host. You need to contact them.
they says that you changes your name server. thats the problem has been happen
You need to clarify that with them. If they require you to have the nameservers pointed to them and if they suspend your account otherwise, you wont be able to use Cloudflare.