Display too large for monitor screen

Something changed all of a sudden, maybe when a game was
loaded for my kids....

One day fine, the next all windows are too large for the
screen. As a result, on some windows I can't select the
options on the right hand side and in some cases am unable
to move the scree left. I tried the Display Settings but
this makes the entire screen way too small even with the
next smaller setting. What do I need to do??