Was being plugged into a computer

No, it's a big mess!

Try to say it another way.

This is my another try :

The monitor that attached to the computer over boss office has been taken by him.

User_garyover boss office

What does this mean?

User_garyhas been taken by him.

This should probably be replaced by putting "He took. . . " at the beginning of the sentence.

I'm guessing that you want something like
"He took the monitor that was attached to the computer . . . " but I have no idea how the boss's

Thank you.

I have heard people saying "over here", "over there". Similarly, I wrote, "over boss office" . Actually I meant "in the boss's office"

Ah. So what you want to say is "He took the monitor that was attached to the computer in the boss's office."

(You might hear someone say "over in the boss's office" -- that would emphasize that the boss's office was relatively far away from the speaker -- but you still need "in". And you certainly don't need "over.")