Please help...Corsa 51 Plate Rear Coil "I think"?

Hi There
My car Corsa Comfort, 61,000 miles was parked at a friends all day yesterday, then all of a sudden whilst she was outside she heard a loud noise coming from where she thought was my car - The sound of metal hitting the ground...She then found a metal coil shape part by at the drivers rear of my car (It wasnt a whole coil - just one complete circle...if that makes sense). I drove my car home, and it felt ok...I think. Can someone help - Im a woman who knows nothing about cars!! - is it ok for me to still drive my car to the garage etc (16 miles away).....Dont want to do any further damage. If it is the rear coil - is it expensive??? I hope not, Ive only just sored out the gear linkages 2 weeks ago!! Many thanks