Thank you for contacting Playerunknown's Battlegrounds!
There are many ways to secure your account, but I'll be happy to let you know about common methods.
1) Do not share your password.
Basically, sharing accounts with others is not the recommended method.
Password sharing is dangerous because it is difficult to compensate for the damage that can occur after sharing the password.
2) Steam Guard installation
Mobile Authenticator needs to install a dedicated app on your phone.
Please refer to the link below for a detailed guide on installing the mobile authenticator.
This information should help in getting back to the game.