My daughter is having suicidal thoughts, what can I do?

She's 23, doesn't live with us. She cut off relationship with us a year ago. She's been resentful since she was 14. She went through a bad phase, made mistakes, got disrespectful, a bit mean to her lil sister and we had to be somewhat harsh. She apologized, but we said we couldn't trust her at that time. Today I think she was being sincere, abut at that time we thought she had to learn from their mistakes. This broke her spirit, she never got over that. Fortunately she changed, never messed up again, but kept her resentment, never got over, never saw us as loving parents anymore, though we tried hard. Today she's graduated, still single, has a good job, is very competent, everybody says she's a great young adult, has a sweet heart. But still has serious psychological problems, since we punished her. She doesn't talk to us, but some friends told us she's having suicidal thoughts. Is there anything we can do? Despite her resentment we love her dearly, we're really scared.