Press and hold the Power button
Tap Restart if it appears
If the screen is frozen, press and hold Power + Volume Up until the phone restarts
If that does not work, press and hold Power + Volume Down until the phone restarts
If the battery is removable, turn off the phone, remove the battery, reinsert it, and power it on
If the phone is unresponsive, let the battery drain completely, then charge it and turn it on again