ThinkCentre M90z not booting up, power led lights on and off

Hi everyone:

My children have been using the M90z ThinkCentre  (Model Number RT1; Product ID: 0800RT1)  for online school and the touchscreen feature has been absolutely terrific for my youngest who is 6 and whose online learning tools make full use of the 23 inch touchscreen.

Regrettably, the PC is no longer booting up since last Sunday.  My son was watching a video on it Saturday night without incident and we proceeded to shut it down, again without incident.  The next day, my daughter tried turning it on and the power button would light up and then turn off and then go over this cycle continuously until the power button was pressed until the PC shut down.  The BIOS screen would not be displayed at all and could not be accessed.

I've tried the following without success:

1.  unplugging, pressing power button for 60 seconds without the PC being plugged in , waiting 5 minutes pressing power button for 30 seconds and then replugging the power cord and trying to turn the PC back on;

2.  opening up the PC, removing the CMOS battery and repeating the actions described in item 1 above;

3.  removing and reseating the RAM;

4.  removing and reseating one of the two 4GB sticks of RAM;

5.  opening up the PC, removing the RAM, HDD and DVD reader;

6.  swapping out the power supply unit from that of another M90z, this one functioning (different model but uses the same PSU) and trying to turn on the PC;

7.  repeating steps 3-6 with alternate PSU from other M90z;

8.  switching RAM sticks from the other M90z (2x2GB);

9. repeating steps 3-6 with alternate RAM sticks; and

10.  switched the BIOS recover jumper to the maintenance position and pressed the F12 key repeatedly when turning on the PC;

A few observations:

1.  The computer hangs before the BIOS menu option becomes available.  Actually, nothing is ever displayed on the screen prior to freeze-up.

2.  The connection light on my USB keyboard does not turn on when I turn on the PC.

3. The PSU and CPU fans cycle on and quickly off upon each attempt;

4. When I take out the RAM, I hear the 4 beep error code so the Motherboard recognizes that the RAM is absent.  After 10-15 seconds the PSU fan and the CPU fan then start spinning at full speed (for both the original PSU and swapped-in PSU) and continue to do so until manual shutdown.

5.  I checked and there does not seem to be any damage or anything dislodged, all the connections and the ground wires appear to be properly and securely affixed and the interior of the PC is almost dust free (no clogged fans or anything which I guess is the benefit of an all-in-one that sits on a desk as opposed to PCs that sit on or near the floor).

6.  No recent updates to Windows 10 (last one was in late December) or to the PC's BIOS (last one dates back several years and was prompted by Lenovo app installed on PC).

7.  The only change made to the original PC was to swap out the HDD with a 256 GB SSD but that was 4 years ago and the SDD is working without problem.

8. Some times, the PC stays powered on but nothing happens. Keyboard does not activate, cannot access BIOS menu via F1/F11, cannot boot from USB, no error message displayed on screen, etc.

Again, this PC was operating without any problem and this all came quite suddenly and without any warning.

Any help would be appreciated.