I’m using DietPi headless mode via dietpi-config option.
While ago I made little mistake in setting sshd config, so I was unable to make connection with ssh.
I was able to connect my raspberry pi locally.
But cuz I set headless mode in config, I could not get any screen via HDMI port.
And I’ve set turn all led off, I couldn’t sure whether machine is working or not.
Luckily, I could reconfig sshd setting with blind typing and could establish connection with ssh at that time, but it was harsh experience.
So like dietpi-software, is there any CLI command to use in dietpi-config? to turn off headless mode?
Yea, probably you have disabled the HDMI port from the boot. Enter DietPI-config and enable it under display options
This is eaxtly the issue, OP was not able to enter anything as screen was already disabled. As well SSH was broken 
@ninthsky
Following was/is stated if you try to disable HDMI. It already gives you information on the emergency procedure.
Re-enabling HDMI requires a reboot. If you need emergency HDMI output, edit the following files on first partition of the SDcard from external system:
- In config.txt, set “hdmi_ignore_hotplug=0” and comment/remove all (max_)framebuffer_(width/height) lines.
- In dietpi.txt, set “AUTO_SETUP_HEADLESS=1”.
@Joulinar oh what a stupid, I missunderstood the question 
Because normally use it with headless mod, I disabled HDMI port in dietpi-config after installing on purpose, to save some resources.
But for further cases, think batter enable it.
Thanks.
I think I miss that part before.
Thankyou.