Hijacked not hacked. 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://gtm.you1.cn/storesteam/twofactor/manage
4. Change passwords from a trusted/clean device.
5. Generate new backup codes for your Mobile App https://gtm.you1.cn/storesteam/twofactor/manage
6. Revoke the API key https://gtm.you1.cn/dev/apikey (there should be nothing in the APIKEY)

Regarding items:
https://gtm.you1.cn/helpsteam/faqs/view/3B6E-B322-2400-8D24