On most Marlin boards, you can use M997 to reboot/restart the microcontroller:
M997
While this is technically the command to update the firmware, on most boards this is implemented by a simple reboot, which will load the bootloader which could update the firmware e.g. if there is a firmware.bin file on the SD card. Since unless a firmware update is intended there is no such file on the SD card, the M997 command will just reboot the board.