Q: Screen and projector problems

This morning while using a projector I have used before only the screensaver would project. Things would appear normally on the MacAir screen but only the screensaver would appear through the projector. It is a screensaver I have had for  a long time and a projector I have used before. I tried all my usual approaches to fixing the problem but nothing worked. I am guessing I have accidently changed a setting somewhere but I don't know where.