my email has been changed by someone

[Gotgam]

I am experiencing a very weird and confusing problem so I need your help desperately. So several days ago, an email from steam has been sent to me saying "The email address associated with your Steam account has been successfully changed." Because I was too busy, I had no time to deal with it until today. When I checked my email account today, my original email account has been replaced with a very suspicious email account ending with .online. Strangely, I managed to log into my steam desktop app (with Username&password). To change my email, I clicked 'change my email address" and since I forgot my Steam Guard Mobile Authenticator code, did not set up my phone number for the account, and don't know what the changed email is, I needed to verify my password. Even though it showed my correct username and I put in the same password that I used to log in, it said 'The email address or phone number you entered is invalid. Please try again.' Does anyone know what to do??

[Cathulhu]

Hijacked, not hacked. You leaked your account credentials somehow.

Do not trade until your account is secured.

Take the following steps to secure your account:

1. Scan for malware. https://www.malwarebytes.com/
2. Check that the email and phone number on the Steam account are still yours.
3. Deauthorize all other devices. https://store.steampowered.com/twofactor/manage
4. Change passwords from a clean computer.
5. Generate new backup codes for your Mobile App. https://store.steampowered.com/twofactor/manage
6. Revoke the API key (there should be no key). https://steamcommunity.com/dev/apikey

Steam does not return inventory items or wallet funds: https://help.steampowered.com/faqs/view/3B6E-B322-2400-8D24

If you no longer have access to your account, read this:
https://steamcommunity.com/sharedfiles/filedetails/?id=1126288560