Thank goodness it's up and running again! 
Yesterday, around 10 in the evening, the screen went black... I tried to revive it but it only made this sound as if it's been sleeping and was waking up. Nothing happened....nada... I hade a conversation with one of my cousins and she advised me to let it rest during the night. Nothing happened this morning either, same small noises - I'm alive but... 
Before the gym I went to a local computer store, Digital Inn, who is a certified Apple store. I got great advice... and if that didn't work they could help me out and it wouldn't cost a fortune. 
I only did 45 minutes cardio today, didn't sleep well last night - a bit worried about the computer, even if I tried to tell myself that there's nothing you can do right now, and worrying about it will not make it better, but I couldn't help worrying a bit anyway. The lack of sleep and allround tiredness made the workout a bit more tiresome than last time.
When I came home the first thing I did was to shut down the computer, by forcing it to shut down... Then I restarted it again and at the same time held down cmd+r, a recovery command. And it restarted and came to life.
High Sierra has some buggs, I think... First times the computer came alive  I needed to lighten the screen, and the same thing when I woke it again after it fell asleep. It was rather dark at first. The new program 'photos' are more advanced. More possibilities to do changes, not just sharpness, light, colour and details... but also white balance, levels and curves. That's as far as I have come right now.
I have one filter and one overlay on the photo. The overlay is called 'grace', I thought it suitable, I am so thankful my iMac is working again.