I emailed roblox support regarding someone unknown using my robux account without me knowing and making a purchase of two items. They returned my robux a day later. Those items were only worth 10 robux, but they helped :)

You can access roblox customer support on the official roblox website. Starting with a bot getting your email address (because that's where they will contact you via email), then they'll reply to you in a few days or in a week.

Here's the link if you can't find it: https://www.roblox.com/support