How can I fix my Citreon Xsara Picasso?

Whilst driving home I noticed a strange sound when I was breaking and whilst going over speed bumps etc. When I got home I had a look under the car to see if I could find anything and discovered the large plastic panel under the engine bay is loose. The section of panel towards the rear of the engine is effectively dragging on the road, hence the sound effects? Is this easy to fix or do I need a mechanic?