iphone 16e restarts and shuts down and start by itself

I have an iphone 16e that I recently got back from mailing it in to Apple. When I got it back somehow they removed the back casing cover i had on my phone. They didnt do anything to fix it first off and when i did the reset myself the phone got worst than before. It would constantly get stuck and then restart by itself. i can't even get to where I can use the phone and get into settings because it would immediately restart by itself. I have warranty until 2027 of march. i need some guidance on what to do with this. I don't want to have to reset it on the computer again becuase it would just get stuck and freeeze in the middle of an update on the computer. Are the iphone 16e bugged out and full of mysterious glitches or did the person who handled my phone when i sent it in screwed it somehow/ ipIi