Why does my Laptop shuts down fully randomly?

Thanks for your help, I got the same critical error in Even viewer, here it is:

The system has rebooted without cleanly shutting down first. This error is caused because the system stopped responding and the hardware watchdog triggered a system reset.

If you know what that means then please let me know, because what i understood from it is that it's some sort of hardware issue?

@Abdullah Hafiz

What are the event ID and source numbers of the critical error?

Try getting into BIOS (press F2 and then release when laptop first starts) and check if it shuts off when in BIOS.

You may have to wait awhile etc.

if it is stable in BIOS it most likely is not a hardware problem as the only difference is that there is no OS loaded.

if OK try running an older version of memtest86 and check if the laptop stays on OK.

The idea here is that you're not testing the memory so much as testing the rest of the hardware as well to see if it stays powered on.

With getting into BIOS not much is happening as far as hardware goes but the memory test is utilizing other hardware systems on the motherboard.

Just eliminating things trying to narrow it down.

It is not stable in bios settings either.

I would love to give you more details about the critical errors, but now the laptop doesn't even go to windows welcome screen without shutting down

Ps: I noticed that everytime my laptop will NOT shutdown, the fan is spinning a bit loud, if it's spinng slowly, it will most likely crash.

@abbody1337

I realize that you said that you have a cooling pad and a thermal check program but I would still open the laptop and check that the fan is clean and dust free and also that the CPU thermal paste is OK and hasn't dried out and needs to be refreshed. If it has never been refreshed then after 6-7 years it really should be.

Here's the ifixit Toshiba Satellite C55-B5200 Fan Replacement guide that shows how to access the fan and here's a video that shows the CPU (5:15 minutes into the video]

Here's a video that shows how to remove / replace the thermal paste on a CPU.

The reason I suggest this is a loud fan is indicative of something getting too hot and needing to be cooled rapidly and the only thing in a laptop that gets hot is the CPU.

If it isn't cooled to within its normal operating temperature range the system will shutdown to prevent it being damaged.

I may be wrong about this but at least you know that if it still happens after you've done this then it is not a case of a dirty fan or dried thermal paste.

It could be a temp sensor problem but that is inbuilt in the CPU although I think that there are some on the motherboard as well but I don't know about Toshiba though.