Do you have the 7″ touch display ? You probably had to configure the Pi to use the DSI output my modifying the bootloader configuration (the config.txt file on the boot) partition. To switch back to HDMI, you'll need to remove or comment the lines that enable and configure the LCD.
There might be a way to enable the LCD display only of HDMI is not detected through the use of conditional filters in config.txt, but you'll have to experiment and see if that's possible. If the LCD pannel cannot be detected at boot by the bootloader, then conditionally enabling/disabling it might not work.
Please do not post a support request without first reading and following the advice in https://retropie.org.uk/forum/topic/3/read-this-first