Here are the steps to restart your Samsung Galaxy S21:
- Press and hold the power button on the right side of the device for a few seconds.
- A menu will appear on the screen with options to power off, restart, or call emergency services.
- Tap the restart button to restart your device.
- Your device will then reboot and restart.
Alternatively, you can also simulate a restart by pressing and holding the power button and volume down button for about 10 seconds. You should see the Samsung logo appear on the screen and then release the buttons.
Note that, depending on the model and year of your Samsung Galaxy S21, the steps may vary slightly.