Press the two-overlaid-rectangle button (or the square button, or rectangular), to access the recent apps list -> tap the X icon or swipe right or left over the application you want to close
If you want to close all applications at the same time, tap the round button with three parallel lines.
Pulling out the battery when the mobile device is on is one of the best and easiest ways to close the applications that have become unresponsive.
If your device's battery is removable, remove it while your device is on and reinsert it after 20-30 seconds.
Note: No data will be lost during this process. If your device's battery is not removable, check out the other solutions on how to restart your device.
If an application became unresponsive and you cannot close it, and you have access to the Settings, you can force-stop it.
Select an application from the list -> Tap on Force stop
If you are forced to restart your mobile device too often, and your problems still remains (you are still unable to close some applications), consider hard-resetting your device.
To learn how to hard-reset your device, check out the don't know how to reset my device solutions.
If you have exhausted every option to close an unresponsive application, restart your mobile device.
To restart it, press and hold the Power ON/OFF (Sleep/Wake) button and the Volume Down (-) button until the Android (or manufacturer's) logo appears.
Note: You won't lose any data in the process.
Maybe your device has a more serious problem for which you are not aware of. If you tried everything but nothing worked, take your device to a repair store. They will diagnose and fix the problem.
Maybe a virus prevents your programs from closing properly. The best thing you should do is to scan it for viruses.
To learn how to check if your device is infected with a virus, check out the viruses/spywares/malware solutions.
If you are not sure which antivirus software to install, check out the don't know which antivirus software to install solutions.
Maybe your mobile device is slow and prevents the application from running properly.
To learn how to improve your mobile device's speed, check out the mobile device is slow solutions.
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