Please share the system model and operating system installed.
Please share the following information:

1. Does the laptop automatically restart after shutting down?
2. Does it happen while running a specific application?
3. Does the laptop overheat?
4. How frequently does it shut down and restart?

Please observe the laptop in BIOS for 1-2 hours. To access BIOS, press F2 key 2-3 times as soon as the Dell logo appears.

Hope this helps.
Please revert with the findings.

Thanks and Regards,
Manpreet C