Take a deep breath and close your eyes for a moment .
1 try to inspect the signal cable from the two machines PC/Laptop and Projector for properly connected and no damage on the line.
2 Try to reboot windows in order to release the output signal again to projector
3 if two of them above did not work for you ,then
it might be a hotkeys to release the output signal which is different from manufacturing to manufacturing say Acer is not the same as Asus or Dell for instance.
So,what is a hotkeys ?
Press Fn+F5 to toggle display output on the screen for Acer Laptop.
For other Brands would be similar try to look for the symbol on the front of the keyboard would see some similarity icon like this
For Dell would be Fn+F8 to switch display output between the display screen ,external monitor and both.
if all of above methods did not work for you I am sorry for that try to call someone who has a good skill about the subject like IT section or computer technical person to help you.
Another case would be having the video but no audio or vice versa .
This problem in most case would be the input and output ports with the properly connected cable and type of the cable as well or even loose or cut or damage.
How to fix in the future I would recommend to use HDMI cable with both video and audio will come along together in the same cable line and with no loss signal from the original PC or Laptop.
However ,your PC,Laptop and Projector must have this HDMI Port mounted .
Have a look and check if your devices have HDMI Port available to use or not.