Today I've updated a Mac mini and it won't boot even to recovery mode Monitor is switching between being black and turned off. The same when starting with Cmd-R. I clicked update, rebooted computer, and this happened. What can I do? I was updating to newest El Captain, probably, as I already had quite fresh version of it.
After I disconnected power for a longer time (to get the serial number), ~25 minutes, the system started to recovery console, and then started normally. I did SMC reset via 15 second unplug, but apparently this was not enough.