not booting after dist-upgrade a few days ago

[chesty]

I dist-upgraded raspian a few days ago, it updated the kernel I think, lots of messages about files in /boot being diverted. After rebooting it didn't come back online.

I've tried plugging a monitor in, but the monitor doesn't detect the pi is plugged into the hdmi port. This was untested before, it might be a dodgy cable or something, I'm not too worried about that.

So, how do go about debugging this without being able to see what's happening?

[chesty]

very strange.

I set
hdmi_safe=1
still nothing on the display
then I set
config_hdmi_boost=4
and i didn't get a display every time, but I did get a display once, and it was a noisy screen with an picture of a raspberry and what looked like a kernel prompt, i think it had trouble finding or mounting /
I plugged a keyboard in, but it didn't work. rebooted again, this time display was blank.

then I commented hdmi_safe and commented out config_hdmi_boost=4 and I it booted normally

It's working now.