To get back hidden games on Steam, navigate to View > Hidden Games in the Steam client.

Then, right-click the game you want to unhide, select Manage, and then Remove from Hidden.
Steps to unhide a game on Steam:

Open Steam: Launch the Steam application on your computer.

Go to Hidden Games: Click on View in the top left corner, then select Hidden Games.

Unhide the game: Right-click the game you want to unhide, then select Manage and finally Remove from Hidden.

You can also unhide games by right-clicking them in the main game list and selecting Manage > Remove from Hidden.

https://www.youtube.com/watch?v=c3PvvpPun_Q

If the above suggest by magicISO Sweden doesnt work for you, it is possible the hijacker deleted your games..

Try the following:

https://steamhost.cn/help_steampowered_com/en/wizard/HelpWithGame
Enter name of the game in search bar,
Click on game,
Click on "it's not in my library",
"Restore removed package".