Sudden shut down

You probably have a clog of dust between the fan and the heatsink. It would be best to have it disassembled to clean, although a few models have clean-out doors to the fan on the back. You might be able to somewhat clear it by blowing canned air in the vent, but this may push dust balls into the fan that go right back against the heat sink.

If sol 1 didn't work,
shutting down while not working on it?

-1 check if power saving feature in BIOS is set to three minutes
and change that to more time- or disable it fully

-2 check battery capacity - could be low or suffering from memory-effect
- let it run without battery, just the cable and wait for the issue again:

if yes goto procedure 1 again

get the speedfan-program
that displays cpu temperature and fan frequency in task-leiste
gives ability to even speed up fan an cool down cpu !

You need to clean it out with some compressed air.

Make sure the laptop is turned off of course.

If you still have heating issues, then you'll need to take it apart and put a new layer of thermal paste on the heat sink / cpu.

Best of luck!