This is an old bug, but i don't remember if i've mentioned this:
If MiSTer is powered BOFORE TV/Monitor, then most likely you won't get the video output on HDMI. Even pressing reset won't make the video working.
The only way to get a video output is to power off the MiSTer and then power on again.
Power and connect the TV/Monitor before powering the MiSTer.
Probably it's because of config i send to HDMI chip. I hope i will fix it later.