You didn't give any clues about what it is so its hard to say.
Here's the thing. To your parents you will always be their little girl, and parents hurt when they see their children hurt. And what hurts more is seeing that something is wrong but not knowing what it is and not being able to do anything about it.
If you have put it behind you, just tell them in brief what was going on, tell them it is over now and you're ok. Give them some peace.
Otherwise they will be worried about it for a long time.
hope it helped :)
Stage a mock hostage situation and demand a helicopter, blank passports, and a bucket of red vines. By the time the police realize there was no situation your parents would just be happy that you are safe and will have forgotten all about it.
THEY'LL FIND OUT IN THE END.