stuck on restart
2019 iMac 21.5 inch running OS Catalina 10.15.7. when I click on restart in apple menu the screen go blank and nothing else happens, the Mac won't restart. I press the on/off button at the rear and iMac restarts. Also on occasions I get the spinning rainbow wheel, why is this happening? Do I need to reinstall Catalina?
iMac 21.5″, macOS 10.15