MacBook Pro connected to projector image will not appear on screen what to do?

Good morning, davevez,

The resources below will guide you through the process of connecting an external display or projector to your MacBook Pro.

OS X: How to use multiple displays with your Mac in Mountain Lion and earlier - Apple Support
https://support.apple.com/en-us/HT202351

Quick key commands for video controls:

• Press Command-F1 to toggle between video mirroring and extended desktop
• Press Command-F2 to detect displays

How to use your Mac notebook computer in closed-clamshell (display closed) mode with an external display - Apple Support
https://support.apple.com/en-us/HT201834

Apple computers: Troubleshooting issues with video on internal or external displays - Apple Support
https://support.apple.com/en-us/HT201177

Hopefully, this information, along with the manual for the projector you are using, will get you on track.

Have a good day,

- Judy