Steam Account Hacked

They would need to have a key logger installed on your pc or have phished your email info.

If there is a key logger on your pc you got more than your steam accounts hacked. Your bank info, all email accounts, all your game accounts, admin login for remote access to your files, the list goes on. If you know nothing about internet security you may want to hire a professional to clean your pc. If they just phished your email logins you have saved to your browser you just need to avoid the phish site and change all your passwords. You can prevent this in the future by using ccleaner after browser use and not storing your passwords/logins so they aren't automated -unphishable-. After you got all this in order submit a support ticket to steam and recover the accounts.

Reasons to hire a professional- If you said, "I have an antivirus and it says I don't have a virus" Its more complex then your -bs- -av- software. Cleaning a computer takes special setup procedures- disabling emulators- unhiding protected files scans from multiple av systems, some times manual removals it takes me about 8-12 hours to clean a pc properly and make sure there is nothing in it, and defaulting all your browser data ect. Recent computer I fixed took 11 scans from 6 different av software to fully clean it then all the damage to the permissions, firewall permissions, browsers, remenants left in the registry, defragging, scheduled tasks, open ports, remote ports, ect ect can really drag out the process. If your clueless take it microcenter or something--

If you just got fished- stop storing info in your browser and change all your passwords then contact steam.

-Note: I would like to add to this if you have been keylogger'd your entire identity is compromised, they have enough info to get credit cards, change email passwords, withdraw all the money from your bank, get ss cards birth certificats license/state id's and assume your entire Identity. If it was a keylogger it is going to take you months if not years to fix the problem. You will be on with account support to get your emails back, you need to contact your bank get access to your online banking change your account number and monitors banking transactions, You need to buy a credit monitor service to monitor your credit for people opening credit lines in your name, watch all grid data daily till your entire grid data has different account passwords and account numbers. I may have missed some stuff.

Since I am online so much I do the following-
Never store personal data and remove personal data from servers for games, companies, and purchases I make online.
Clean my pc regularly
I have a credit monitoring service that keeps track of my credit lines for me and alerts me when a new line is established
Keep track of all my banking data on biweekly statements.

Use different passwords for different sites that work off the same network- Example
team- different password
MPGH- different password
The game site- Different password
Youtube- different password/email
Other hack sites- different password
Online storage for hacks- different password
Banking- Different password/email
Paypal- Different password/email
However these websites are all different locations they are all used together network and used with steam if one gets compromised the passwords are all different so the entire network doesn't fall to one mistake. It does require memorizing more passwords but its worth it. 1 Keylogger would compromise all of them. Now if one of them got phished they would only have 1 password/ 1 login and not be able to loging every site with same login and password they got from steam or youtube. I also have silent email accounts and cell phone numbers linked to my emails and game accounts, the silent emails let me recover hacked email accounts and the cell phone lets me recover them also. Authenticators attached to accounts is useful to providing the authenticator data doesn't get stolen even then they would need to get your login and pass still.