Gas Cap blows off

edit: After more questions I think the spring rod in the trunk broke and my wife had left the gas cap and door open.

Wife's driving alone and hears this bang like a gun shot and looks in the mirror and see's the gas door open,stops car and cap is off. Later opens trunk and find tension rod that hold lip open broken. I' don't know... Any body got any suggestions on whats wrong here? Thanks. Its a 1999 225000 miles.