Face ID phones (iPhone X and newer): Press and hold Side + either Volume button until the power-off slider appears → drag it → after it shuts down, press & hold Side until the Apple logo.
Older iPhones (with Home or top button): Press & hold the Side/Top button until the slider appears → drag it → press & hold the Side/Top button to turn back on.
2) Force restart (if frozen / unresponsive)
iPhone 8 and later (including iPhone X, 11, 12, 13, 14, 15...): Quick press Volume Up, quick press Volume Down, then press & hold Side until Apple logo appears.
iPhone 7 / 7 Plus: Press & hold Side + Volume Down until Apple logo.
iPhone 6s and earlier (with Home button): Press & hold Home + Side/Top until Apple logo.
If it still won’t restart, try charging it for 15–30 minutes or contact Apple Support.