Black screen - imac - nvram deleted, safe mode not possible

Hi,

my imac, 21,5 inch, mid 2010 shows me the black screen after 1 second white screen after pushing the power button. Updated recently to Sierra (can‘t remember which one exactly but did the latest update a few days ago).

After the first time this happened, I deleted the nvram and everything went back to normal, I made a backup via timemachine and thought problem solved, yesterday it worked as usual.

Today its only the black screen again, with said white screen for 1 sec., deleting nvram doesn‘t change a thing, safe mode neither. In the background the imac seems to work as usual, when I pushed the power button just shortly I saw the usual screen for 1-2 seconds, I was able to login without seeing what I did, the next time I saw my usual desktop for 1-2 sec., but it just worked maybe 3 times, then blackness again. Plugged everything else off, changed the socket, can‘t find any other solutions...

thanks for your suggestions!