Whether it happened today or 4 years ago, I would seek out help for her. Usually people cut themselves to release the pain; therefore feeling better. If she hasn't resolved those issues, then there is a chance it might happen again.
First thing you need to do is hug your daughter. Tell her how much you love her and you would hate for anything to happen to her. I would also apologize for not seeing the signs. This is not to make you feel guilty, but to let her know that you really do care. Like you said you didn't see the signs and chances are she was probably very good at hiding them. Reminder her that you will always be there for her. then listen. Listen to what is troubling her. Listen to the good things she shares with you as well. When she is talking to you, really pay attention. Stop what you are doing, face her and listen. If you are in the middle of something where you can't give her your full attention, then ask her if you can do it at a convenient time for both you like after dinner. Keep the lines of communication open. She might not tell you right way why she was cutting. Don't push, just be there if this happens. A professional can help figure that part out.
The next thing you may want to do is get her professional help. Some children feel more comfortable talking to someone other than a family member. Many towns have free or low cost counseling services. Also, check with you family's health insurance company for what is covered. One more place to check is your employer. Many companies today have employee services that include mental health.