By default, if a HDMI is not connected to the Raspberry Pi at boot, then if one is plugged in later, it will not be recognised until the Pi is rebooted. This is to save power consumption in applications where HDMI output is not required.
This is not ideal for certain applications, such as using SnapCamera and then viewing the images that were taken.
To disable this feature, edit the /boot/config.txt file in your favourite text editor
sudo nano /boot/config.txt
And uncomment (remove the # at the start of the line) the line