Solved. I had to reset RYZEN MASTER where I had set it in 'Gaming Mode' to "auto overclock", I think I had to EXCLUDE MEMORY from the auto overclock (involved a convoluted sequence where I had to clear CMOS (button on back of motherboard), then re-enable the XMP Memory Profile for 3600 mhz, then reboot, then restart Ryzen Master, select the Gaming Profile, click "EXCLUDE" on the memory portion, then click RESET, then reboot, and bingo, the 16 cores reappeared. Not exactly a can of corn! Thanks for trying to help me, Martin. Appreciate your time and effort very much.
I did a little more reading on Ryzen Master. There are two modes "Creator Mode" and "Gaming Mode". Gaming mode by default shuts off half the cores to enable "Legacy Mode" (for certain older games like "Dirt", which cannot support more than 10 (?) cores if I remember right, and I was unable to turn "Legacy Mode" off in Gaming Mode within Ryzen Master, so every time I enabled automatic overclock, it killed 8 of my 16 cores . Once I figured it out, I reset "gaming mode" to default (No Overclock) then I went into Creator Mode and enabled auto overclock and restarted my PC and bingo. In Cinebench, all 16 cores lit up to almost 5 ghz (The good news), whilst temps in my 360 AIO Corsair cooling system ratched up to 81 C max which is the absolute top end of heat (normally in flight sim 2020, it runs about 55 C even overclocked. So I won't be running any benchmarks for any long period of time anytime soon in overclocked mode! I surmise that the last mode you were in within Ryzen Master is your 'current' or 'active' profile. Hope this helps others who may be struggling with trying to auto overclock using Ryzen Master. AMD says up to 85C is "acceptable" for their 5000-series Ryzen CPUs, though again, I personally am not that comfortable with anything over 75C. Ryzen Master has a number of daunting 'WARNINGS', I think AMD hired a guy who used to label cigarettes! (May cause permanent damage, void your warranty, early hair loss, and certain unmentionable dysfunctions including but not limited to marital relations.)