It was not hacked, you were phished. You most likely either:

- Went to an obvious scam site and entered your details there

- Downloaded malicious software

- Engaged in malicious activities with an obvious scammer.

- Purchased a banned account and now you're trying to get the ban removed

Account security is your responsibility at all times.

Hijacked. Malware or phishing.

Follow steps 1-6 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://psteamproxy.yuanyoumao.com/twofactor/manage
4. Change passwords from a trusted/clean device.
5. Generate new backup codes for your Mobile App https://psteamproxy.yuanyoumao.com/twofactor/manage
6. Revoke the API key https://psteamcommunity.yuanyoumao.com/dev/apikey (there should be nothing in the APIKEY)

Regarding items:
https://psteamhelp.yuanyoumao.com/faqs/view/3B6E-B322-2400-8D24