Problem with a IMAC 27 2017 3.5. 24 GB

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I have a 7 year old IMAC 27 inch,Lately it has got frozen i press all the keys and nothing happens ,I have to switch it off on the Mac.But the last time it turned it self off the screen went black ,i kept pushing the keys ,it came to the blue screen ,i had to turn it off from the MAC .Is it time i got a new Mac?I was hoping it would last longer than this.There must be a problem with it.It is the modal with a SSD in it.Plus i all so have another external with another OS on it and the same thing happens there.Any idea what the problem is?I put this on a Apple forum and they said it could be the PSU falling.But the price of Apple stuff might mean it would cost to much to repair.I left it on for four hours it was making a lot of humming noise .i pressed all the keys but nothing happend .I was ready to transfer to my Windows PC.I unplugged it from the wall.I plugged t back in it is working again and no noise. It is a no no to take it in to a the so called Apple genius.