I would say: どうかしましたか。 何かお困りですか(なにかおこまりですか)。 We use the formal form to unknown people and there are several of them depending on the age of the speaker and the unknown person. So if she/he is a child, I would say どうしたの？ / 困ってることがあるの？ / 道がわからなくなっちゃったの？ etc. Generally, we use お手伝いしましょうか when someone need help "physically", for example, carrying a big bagage, fixing something, helping people to go up steps, etc. I hope this was helpful.