My laptop os shutting down after every 20mins

A laptop shutting down every twenty minutes is likely caused by either:
1. Running from battery, not mains, and battery only lasting 20 minutes, or
2. If running from mains or battery is not discharged, it may be overheating and shutting down to protect itself from damage due to excessive heat. This is more likely at the laptop is old. The solution is to power off, remove the battery, clean the fan completely, check for any damaged components such as capacitors which may have bulged, indicating not working correctly.
3. If the fan was clogged or very dirty, clean and replace the battery, connect the mains and try again, it should last longer than twenty minutes. You may be able to find a new battery to replace the worn out battery.
I run some very old laptops without a battery since I can run from the mains.