Here are some things you should try if your laptop is not connecting to a projector after changing the display resolution:

1. Ensure your display cable is connected to the correct port on your laptop and the projector.
2. Check the display settings on your laptop to ensure it is compatible with the projector’s display resolution.
3. Check the power of the projector and the display source to ensure they are both powered on.
4. Toggle the display source and power off the laptop, wait several seconds, then turn them both back on.