Policies can be changed. Unless you have a binding employment contract or collective bargaining agreement guaranteeing you a raise after 6 months, your employer doesn't have to give you one. You will not win if you sue.
Be polite when you ask. Just mention that your understanding was (you were told) that your job policy was you get a raise after 6 months & you were just wondering if the policy still stands since you didn't get one. (or something on that order)
Not only do you not have a case to sue (would not win), you also "might" not even get a raise. Talk to your mgr. or HR & see what they say. (try your mgr. first) I do hope you get one.