Hi, it may have been better to remain with the original post so we know what has been tried.
Anyway, not sure what you mean by: quote
"Then turned on my PC and noticed I mistakenly disconnected the CPU from the Motherboard."
How can you disconnect the CPU by mistake?
Did you remove the CPU for some reason?
As for the Pentium logo appearing, you may have changed something in the BIOS that now permits this splash screen. Double check your settings and disable this option.
The fact that your computer allows access to the BIOS settings as you have managed to check your temperature settings, suggests this is probably an operating system failure and not a hardware failure that is causing the shutdown issues.
Does the computer boot into safe mode and remain stable?
If you have been "playing" with your BIOS settings, I suggest restoring the default settings.
I appear to be having a senior moment, what do you mean by "IDA." Have you since removed the HDD secondary connection and is the same problem remaining?