you need to contact their legal department who wil then either sort out compo depending on the road that it happened on or pass you onto the company who was supposed to look after the road or have recently done roadworks on that stretch
make sure you explain everything from a dumb non car enthisisats point of view, i called out the aa becaise my car was "making a noise" and then it went on from there

good luck and i'll give you some pointers if you get stuck, been there done that several times now without failure to win, biggest score was £1800