You exposed your login credentials:
a) Either by logging into a site that faked a Steam login and made a bot log into your account using the save password as well as the trust device feature while injecting a Steam API access into it.
b) Or by installing malware that stole your session data or injected a keylogger.
c) Or by using outdated login information that got exposed in a leak.
d) Or by falling for a Steam Support impersonation scam on Discord or similar platforms.

1. Scan for malware https://www.malwarebytes.com/
2. Check that the email and phone number on the Steam account are still yours.
3. Ensure your email address and/or password aren't contained in any public breaches. There are various sites that allow you to do that.
-- If they are contained in any public breaches, change your email account's password from a secure computer before proceeding.
-- If that happens, you may want to secure other accounts than just Steam.
-- Consider using mobile two-factor authentication on your e-mail address if your e-mail provider supports it.
4. Deauthorize all devices INCLUDING the one you are using (You will have to relog after you click it): Steam -> Settings -> Security -> Deauthorize All Devices or https://steamhost.cn/twofactor/manage
5. Change passwords from a clean computer
6. Generate new backup codes for your Mobile App https://steamhost.cn/twofactor/manage
7. Revoke the API key https://steamhost.cn/steamcommunity_com/dev/apikey (there should be nothing in the APIKEY)
8. Change your trade link: Profile > your inventory > trade offer > Who can send me trade offer > scroll down and make a new trade link.
9. If points were stolen within 14 days, reset your Steam password (not change, RESET using Forgot Password) to cancel pending awards.
10. Once you have done all of the above steps, edit your profile to get rid of the fake message planted by the scammer (if it exists).

Steam Support will not restore stolen items nor stolen wallet funds.
In accordance with Section 1 C of the Steam Subscriber Agreement, you are fully responsible for all actions on your account, no matter who used the account. This includes actions that occurred as the consequence of fraudulent account access by phishing or malware, be it input relay, session token theft or any other method that granted a third party access to your account.

The ONLY way you can contact Steam support and through which you will receive official answers is through https://steamhost.cn/help_steampowered_com and no other way.

Anyone claiming to be from Valve contacting you through Discord or other means is a scammer.

Aye man you're a lifesaver thank you, I did lose all of my inventory but I hadn't played CSGO 2 in a while because it's not the same anymore. I followed all of your steps, I'm a United States Marine and one of my friends here that's in data was able to get the malware off of my computer and installed his own programs to prevent that from happening again and yeah the hacker put a fake URL in my summary and I didn't even realize that. I knew it was sus when the "representative" was arguing with me because I called them a re***d, but I trusted it because all my friends were deleted and profile changed, I've never been hacked before so I didn't read between the lines as much as I should have and now I know I will never fall for anything like that ever again smh