Steps to take NOW:
1. Scan for malware https://www.malwarebytes.com/
2. Deauthorize all other devices https://steamproxy.com/steamstore/twofactor/manage
3. Change passwords from a clean computer
4. Generate new backup codes for your Mobile App https://steamproxy.com/steamstore/twofactor/manage
5. Revoke the API key https://steamproxy.com/dev/apikey (there should be nothing in the APIKEY)


Please review how you are logging into Steam, you somehow gave them your log in information. This could of been due to the computer being compromised and redirecting to a fake login, or you using a 3rd party site to login to steam.

Once you are sure you have regained control, you can start trading again. Please triple check any trades you make to be sure they are going to the correct account.

After you have secured the account, please edit your profile back to normal (if it was altered by the hijacker/scammer)