Sandbox happy, now porting to Production..

I been playing around the API for sometime now and I'm happy with what i got.

Now, I'm trying to port to the production server but right away, I'm having problem trying to login.

I'm using Advance REST client and the Authentication modal dialog doesn't go away. (keeps failing. I keep getting the following.

{

"error": "invalid_client"

"error_description": "Access Denied."

}

I tried again with my Sandbox credential and it works fine.

I've double check the URL for both server and double check the endpoints but to no avail.

Please help.

Thanks.