Plug your phone into a charger.There's a chance that your phone has run out of battery and will not turn on. Plug your phone in and let it charge for a few minutes before trying any of the following steps.
Try turning your phone off the regular way. Press and hold the Power button on your phone until the Power menu appears. Tap "Power off" to turn the phone off.
If this doesn't work, move on to the next step.
Force your phone to restart. If your phone isn't responding to your Power button or screen taps, you may be able to force the device to restart.
Most Android devices can be forced to restart by holding the Power and Volume Up buttons for about ten seconds.
If Power/Volume Up doesn't work, try Power/Volume Down.
Remove the battery if you can't force a restart. If you can't force your device to restart, try removing the battery and then reinserting it. This is only possible on Android devices with removable batteries. If this doesn't fix your problem, move on to the next step.
If your Android has a removable battery, you can remove the back cover and then lift the battery out. wait about ten seconds and then reinsert the battery.(How do I fix a frozen phone if the battery in my model is not removable?)
Press and hold all of your buttons on your phone at once and hold for 10 seconds.
Delete apps that are causing your phone to freeze. If your phone is freezing whenever you run a certain app, there's a good chance that the app is causing your system to crash. The easiest way to prevent this from continuing is to simply stop using the app and deleting it.
Perform a factory reset if your phone won't boot up. If you can't get your phone to turn on after it was frozen, a factory reset may fix your problems. All of the data on your phone will be deleted however.
Start with the phone turned off. In order to access Recovery Mode, your Android phone will need to start from a powered-off state.
Hold down the Recovery mode buttons. These vary depending on the model phone you have:
Nexus, LG, Motorola - Power and Volume Down
Samsung - Power, Volume Up, and Home
Continue holding the buttons until the Android boot menu appears.
Use the Volume Up and Volume Down keys to navigate the menu. Select "Recovery" and press the Power button to select it.
Select "Wipe data / factory reset" and press Power. Select "Yes" to confirm. Once the format is complete, your phone will be rebooted and you can set it up as if it was new.