无法在服务器中设置会话，在localhost中工作

I am setting user id to the session on login in my website.On echoing the session variable soon after being set, it is displayed.But after that iam redirecting this to another page where session is strated and then checked for the session.But it displays error message that undefined index user_id.This code is working in localhost,was working in server also.But now it displays error.Unable to login to the website due to the problem in setting session.I am setting user id to the session on login in