Q: suddenly stopped working key of the third row on the keyboard, starting with Q at O

And you are absolutely sure that you have not had a spillage of liquid in that area?   When a run of keys in a line occurs, liquid damage is often the cause.    If you can say 'No', to my suggestion, take it to your nearest apple store and ask them for an assessment.

How long have you had this keyboard; is it a Magic Keyboard 2.