Why is my phone not turning on. it is just showing the restart screen

@shahanmoha67345 try to first boot your phone into safe mode.

• Press and hold the Power and Volume down buttons to power off your Samsung phone.
• Press the Power button to turn on your device.
• Once the "Samsung" logo appears on the screen, release the Power button and then immediately press and hold the Volume Down button.
• If succeed, you should see "Safe mode" appearing in the lower-left of the screen upon reboot.

If that works then the issue is most likely related to a bad 3rd party app. Uninstall those.

Next you could try to wipe your cache.

• Press and hold the Home and Volume Up button at the same time.
• Keep holding those buttons and now press and hold the Power button.
• When the Samsung logo displays, release all three buttons and wait for the device to boot into recovery mode.
• Now you can use the volume down button to go to the "Wipe Cache Partition" option. Select it by pressing the Power button.
• Use the Volume down button to highlight Yes and press the Power button to select it.
• When the wipe cache partition is complete, select the Reboot system now.

If none of that works you can try a factory reset. Do this only if you know your email and password that you've used when you set-up the phone. After that you may have to consider flashing your firmware.

Of course, hardware is always a possibility as well. At minimum replace the battery.