Hi caoimhe, welcome to Marriott Insiders. I thought I had seen some information on requesting a replacement card somewhere on the site, but I have not been able to locate it. If another Insider does not know the answer, a call to Customer Service would probably be the best way to request a new card.
I'm so sorry to learn that you lost your card in a housefire. I hope your other losses were minor.
Once you sign on to Marriott . com and sign in to your account....You can go to "member profile".. On the bottom right will be a bunch of choices.. Click on "Reward Membership".. A drop down box will reveal a link titled..."Order replacement Rewards card".
Once you click on that, a verification message will come up telling you that your card is on the way. Make sure the address listed is where you want it sent.... Or, You can call Customer Service.