my website plugin hacked

Dear forum people,
I own a website contruct via wordpress. Of recent, I observed that some of plugins are not working. I wanted to update them, but an automated message on the screen read that i have no sufficient permission to do this. I tried several time, but not allowing me to access the website setting. It is not allowing me update wordpress to 3.6
Also, I observe that certain websites of business companies create links through my domain name without my authorisation.
Is it that my website is hacked? How can solve this problem. I really need urgent advice. Any help will be higly appreciated.