my phone abruptly switched off and is not restarting

(1) Try and Force Restart your iPhone EXACTLY as shown below and see whether that resolves the issue:

• Press and quickly release Volume UP button
• Press and quickly release Volume DOWN button
• Press and Hold the SIDE button until an Apple logo appears and then release the Side button (Can take up to 20 seconds. (DO NOT release Side Button when invited to Slide Power OFF). 

(2) If the above does not work then - Follow the instructions in this article by Apple ➔ If your iPhone is Non-Responsive or Frozen and see if that helps.

(3) If that does not help your phone might need service ➔ Contact Apple for support and service - Apple Support

Axel F.

Hello poonam_aundhekar!

If your iPhone won't turn on or is frozen - Apple Support, please see this Support Article to learn what to do next.

I hope this is helpful. Good Luck!

"My iPhone 11 abruptly switched and won't start again"

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Being Stuck in a Boot Loop:

Your iPhone might be stuck in a boot loop. This typically happens when you unsuccessfully download something or an update stalls.  So, refer to my User Tip: How to Fix an iPhone in a Boot Loop - Apple Support

When was the last time that you replaced the battery on your phone?