Imac will not go off

My Imac no longer shuts down properly. 
The shutdown stops with a black screen on which the cursor can still be seen. 
From there, the Imac is blocked. 
This also happens on restart. 
And also when you want to install an update for which the Imac has to be restarted automatically. This process is stalling. 
Who has good advice. 
I have already tried everything that came up on the internet with advice and nothing has solved this problem so far. 
The Imac works well. It just doesn't goes off anymore. Unless I pull the plug.
Thank you for your advice.