my pc switches off after using it for long time

This is most probably due to overheating.

Install an app to check the temperatures (coretemp, speccy, speedfan, etc) and monitor them until the pc shuts down.

As someone else said, it is most probably the CPU (nowadays the RAM is pretty much never to blame for anything).

Quickly check if the CPU fan is actually working, if it is then monitor the temperatures.

Start with the simple possibilities first.

Probably the CPU overheating. The thermal paste between the CPU and Heatsink will degrade over time. A simpler answer is that your CPU fan and heatsink are covered in dust, which acts as an insulator. Perhaps your entire PC is covered in dust. One might assume that you clean the inside of your computer every so often, but you didn't mention it, so I won't assume it.

You may also have the bios set to shut the PC down once it reaches a certain temperature. This combined with a dusty system could be the culprit. Check your BIOS under Hardware Monitoring and see if this is the case.

Can't be more certain without additional information.

First check that your RAM strips are clean or not, and they are properly seated in their slots. Moreover, disable the "Fast Boot" option in your BIOS. It also may be due to overheating. I have also heard this as a symptom of overheating. Run speed fan test, and see if the Temperature go up and the computers shut down or touch the heat sink and check if it is too hot to touch

This same thing used to happen with my 7-year-old HP laptop. Try reseating RAM in a different slot(if you have more than one available) after cleaning its pins with an eraser. Otherwise, consider changing RAM, it will most probably solve your problem.