First of all you need to force close the app that is causing trouble, Swipe to reveal the tab-Select the app you want to kill by tapping the stop button. If you are not able to view the app in the given list then go to the upper right corner and look for Show Cached Processes.
You can even attempt a force shutdown hold and press down power button + volume up button till the screen turns off. Power on by pressing the power button. If you are having trouble using the volume up button then use ok google set up my phone the volume down button.
If your Android has a removable battery then you can remove the back cover of your phone and pull out the battery. After a few seconds replace the battery and close the battery cover, now turn back your phone. However, remember to use this method when anything else is not working in unfreezing your Android Phone.
Another way of unfreezing your phone is to try out the method of factory reset. Restoring your phone back to the factory default might help you get rid of the phone freeze trouble. However, before you begin the process of factory reset you need to take a backup of your phone by connecting it to your system via USB.
For factory reset go to Settings-Backup-Reset and locate Factory data reset, tap it. A confirmation is required for proceeding further. After you have confirmed the process of factory reset will begin.
Undertaking the above-mentioned steps will ensure that you rid your phone of the phone freeze trouble. However, if it again happens then you need to get in touch with a tech expert.