If you have access to the account

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


If you have lost access to the account

Account Recovery
Make sure you are completely logged out of Steam before you start the recovery process


https://psteamhelp.yuanyoumao.com/en/wizard/HelpWithAccountStolen
https://support.steampowered.com/kb_article.php?ref=2347-qdfn-4366

When doing an account recovery, do not try to rush thru the process. Trying to rush thru normally results in information being missed and time wasted. Take your time and read everything.

Here is a guide that another Steam user was kind enough to make. It will help you navigate your way thru the recovery process if you are having any problems.
https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=1126288560

Thanks for this, i already logged out all devices