Damage to my car

I left my car in their care while traveling, when I returned there was substantial damage to my car. I sent proof of the condition of the car before I left (I have sold my vehicle so the buyer took photos a couple of days before). They are taking no responsibility for the damage, claiming I need to prove it happened while with them. I strongly recommend you find another valet service. If there is damage - they will not pay for it. I have been a client for years and found the entire experience disgusting. No response to mails and calls and no responsibility taken