iMac (Retina 5K, 27-inch, Late 2015) Freezing and Beach Balling Out of Nowhere!

Restart in Safe Mode per the directions in Use safe mode to isolate issues with your Mac - Apple Support then restart normally and retest.

However, I see no evidence of a backup on this computer. If this is correct this is a MAJOR issue and a guarantee you will lose date. At the very very least you should be using Time Machine to backup. If you are not familiar with Time Machine please read Mac Basics Time Machine

Not too late for Time Machine, it may save some valuable data.

Lets try resetting the NVRAM per the directions in https://support.apple.com/en-us/HT204063

if that does not work please reset the SMC 2-3 times with all peripherals disconnected. https://support.apple.com/en-us/HT201295

we have same model< but less memory and disk, same random reboots and freezes