To perform a soft reset on an iPhone 4S, long press the power button on the top edge of the phone, and release it when the Slide to Power Off appears on the screen. Slide to the right, long-press the power button, and release when the Apple logo appears.
A soft reset is a common troubleshooting method that may solve an iPhone that crashes, freezes, continuously restarts, or cannot make or receive calls. It may also help fix a phone that locks up or an unresponsive keypad. Doing a soft reset does not erase any data on the phone. If the smartphone fails to restart or respond after the soft reset, long press the power and home buttons simultaneously, and release when the Apple logo shows on the screen. The logo often appears after around 10 seconds.
If the soft reset doesn’t work, the next possible solution is to perform a hard reset. Make sure your iPhone is fully charged, and back-up all data on a hard drive, raid system, media or another external device before doing the reset process. It may also be necessary to remove the SIM and memory cards.
To do a hard reset, press Settings, General and Reset, and choose Erase All Content and Settings. This option requires entering the passcode. Other options under Reset include Reset All Settings and Reset Network Settings.