iMac shutting down instead of sleeping

Today my iMac has been shutting down instead of sleeping and then I it doesn't turn on from the power button, to get it working again I have to switch off the power at the wall, turn it back on and then turn on the iMac power button to get it working again.

It's an iMac (27 inch, late 2012) running macOS Mojave version 10.14.6

I have adjusted the energy saver settings so that it doesn't go to sleep as a band aid solution, but would be grateful for suggestions about fixing the problem long term?

Many thanks in advance!