OVH - Hosting, website doesn't work if you add "www" in front

I just moved a website from Gandi to OVH, and the website is working perfectly fine, without the "www" adress. But when you add "www", it doesn't work, as if the www wasn't linked to the host. Do you know how I can fix that ? I don't know what to do, I know it has something to do with dns Zones. It's very important because google currently shows that the website may have been pirated. I tried contacting their support but my phone rings in the void for 30 minutes, and my mail hasn't been answered so far...

Thank you.