Is your teenager in with the wrong crowd?
Your teen used to chat to you about their friends but recently they seem to have new friends they don't talk about, and then you find out that they are hanging around with some kids from a bad neighborhood. Worse, when you ask them about it, they become evasive, refusing to answer your questions. You have to tread carefully here, after all, your teenager has the right to choose their own friends, and not everyone living in a bad neighborhood is bad company.
It may well be that their friends are decent enough, but your child knows what will be assumed and is trying to avoid a possible argument. On the other hand, if your son/daughter has also started truanting school, or staying out past curfew, or generally starting to act out more than usual; their new friends may well be part of the problem.
Be smart about this; if you come right out and try banning your teenager from seeing their friends, they are likely to defy you and see them secretly, lying to you about it. You need the decision to be theirs, but firstly, you need to find out what these friends are really like. Try inviting them over, suggesting they do homework together, having a brunch, whatever low-key approach you think will work best. You need to be able to meet them but not hang around, perhaps offer to give them a lift to school, the movies, or the mall.
The idea here is that you are assuming these friends are decent kids, that your teenager has good judgment. If they are good kids, then you will have avoided making a hasty biased decision based on their background. However, if your teenager is hanging around with a bad crowd they will not want you to meet them at all, at which point you can ask them directly if they are ashamed of their friends, or worried that you will dislike them. Remember, the tactic here is to get your son or daughter to make the judgment themselves.
You may get to meet these new friends and dislike them thoroughly, but you don't actually need to like them; you just want reassurance that they will not get your child into trouble! However, if you do feel that these new friends are a bad influence you need to talk to your teenager calmly about their changed behavior since they met these friends. Try to avoid the word "blame" and talk instead about your concerns regarding school, or late nights out. If you have to get heavy handed because they ignore your requests, then be prepared for a long haul. Teenagers are extraordinary loyal to their friends, and can also feel intense peer pressure, especially if they are hanging around with an older crowd. Your only choice at this stage is to keep talking to your teenager, and hope their common sense reasserts itself.