iPhone 7 is turning off and on automatically. I can't do any

Try a forced restart:

press vol up then vol down and then press and hold the side button till the screen shuts off. Keep holding till you see the Apple logo and then release.

if it doesn’t work, try recovery mode:

press vol up and Then vol down and then press and hold the side button till the screen shuts off. Keep holding even if you see the Apple logo, and release when you see the connect to iTunes/computer logo. Then plug your iPhone to your computer and launch iTunes and click Update iPhone. If it doesn’t work then click Restore iPhone, which will wipe all data on the iPhone.