CPU doesn't shutdown, everytime on startup goes to bios after no signal

Oneday after shutting down(from power menu), monitor went off but CPU kept on. After that day weird stuff started.
  1. On startup, at first cpu fan starts spinning(for 1-2 seconds) and monitor shows "AOC" then "NO signal" then off. 2 seconds after that CPU finally turns on and goes to BIOS every time. (No POST beep)
  2. On shutdown(ALT+F4) shows "No Signal" but CPU kept ON. Working with long press shutdown of CPU.
  3. after sleep also monitor doesn't turn on and shows "No Signal", on disconnecting cable shows "cable not connected".
I also changed CMOS battery and manually reset BIOS. Here is the startup message.
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Cycloid Torus

Posts: 4,803   +1,603
Guesses:
"after shutting down(from power menu), monitor went off but CPU kept on. " - might have stalled while doing an update.

"monitor shows "AOC" then "NO signal" then off " - not getting any signal from video subsystem so when this is recognized, the system throws you into BIOS/Setup

"No POST beep " - unable to 'boot', so boot record (MBR) or UEFI settings are missing in action (bad boot drive? corrupted record?)

Other symptoms look secondary.

Look at boot drive maker's website for self-booting test utility. Test boot drive. Locate other boot media and test system. See download for Hiren’s BootCD at https://www.techspot.com/downloads/6966-hirens-bootcd.html