Phone not working properly
Screen keeps turning black for a few seconds
iPhone 11, iOS 14
Question marked as
Top-ranking reply
Arrange repair
First try a Forced restart
https://support.apple.com/guide/iphone/force-restart-iphone-iph8903c3ee6/ios
If no better
You should arrange a service or repair
https://support.apple.com/iphone/repair/service
Anything other than Screen or Battery may be a replacement phone and the price is in the article.
You can see additional service information within the following article:
Get your iPhone, iPad, or iPod touch ready for service - Apple Support