Recently a thread was locked and I thought it shouldn't have been. I read the guidelines and it told me to contact the mentor, but I don't know who that is. So now what?
Maybe a better option would be to go to the last post in thread, and click on "report" at the left. In the report box, outline your concerns. Your report will be viewable by all mentors.
As George has said, use the REPORT post button and write your reasons there. All the mentors and the admin will see your report. There have been threads that had been reopened before, so it is not unusual for us to do that.