I’ve Been Hacked on Steam—What Should I Do?

Hey everyone, I'm in a bit of a panic here. I've been hacked on both my email and Steam account. The hacker managed to change my Steam email to theirs, and now I can't access my account. The only good news is I still have access to my original email and I can prove that the account is mine. I've changed my email password, but I have no idea how this all happened. To make things worse, they unfriended all my friends. I've had this account for years and have so many memories tied to it. I'm really desperate for help, even though there's no money involved. What should I do next?