It's happened to all smartphone users: your device decides to play dead.
This usually means the screen freezes up and no matter what you press it stays in this state. To make matters worse, it prematurely drains the battery as that static screen cannot be turned off.
Before you decide to toss the smartphone under a moving bus, the following are a few things you can try.
All smartphones have a soft reset option, which is similar to restating your computer. This should always be your first option as your phone's information will be recovered — unless you were working on something when it froze up, which you should assume cannot be retrieved.
Depending on your model, soft resets are handled differently. But these instructions should cover the majority of users:
iPhone: Simultaneously press and hold the Sleep/Wake button at the top of the iPhone and the Home button (the circle at the bottom). Hold both buttons for about 10 seconds and the screen will go black. A few seconds later the white Apple logo will appear as the device reboots.
Android: Most Android phones — from the likes of Samsung, LG, HTC, Motorola, Sony Mobile and Huawei — will perform a soft reset whenever you turn the phone off and reboot it again. Because every phone is different, press and hold the power button until you get a message on the screen that asks you to power down or restart.
BlackBerry: Press and hold the ALT-CAP-DEL keys. After two or three seconds, the display will turn black and your BlackBerry will reset. Another way to do this is to turn off the BlackBerry, remove the battery for 30 seconds or so, put it back in and turn it on.
Windows Phone: Similar to Android, a power cycle will work by simply turning the Windows Phone off and on again (not sleep/wake but full restart). If this doesn't do the trick, turn it off and remove the battery for 30 seconds. Then put it back in and restart.
If a soft reset doesn't work — that is, your smartphone still isn't working like it used to -- then you might need to resort to a hard reset.
Keep in mind, however, doing this will return your phone to its original factory settings, therefore you'll lose all data and any installed apps. This is why it's a good idea to connect your smartphone to a computer to make a back-up of all your info as a preventative measure, or use various cloud (online) services to do this over the airwaves.
If your smartphone takes a removable memory card, be sure to take it out of the phone as there may be data you can recover later on.
iPhone: From the Home screen, tap Settings, followed by General and scroll to the bottom to press Reset. Once inside, select Reset All Settings. This should reset everything on the phone but keep your apps and media. If this doesn't stop the phone from freezing, then try the option that says Erase All Content and Settings. This returns the iPhone to factory settings. You can also do this by connecting the iPhone to your computer via a USB cable and from within iTunes, select your device and click the Restore tab.
Android: This might not work on all Android models, but should be similar between all models and versions (e.g. Gingerbread, Ice Cream Sandwich). Again, if these steps don't work exactly, you'll find the right area with a little bit of experimentation in the Settings area. For most phones, choose Settings, then Back Up and Reset (under Personal), and one of your options will be Factory data Reset. Tap it and follow the onscreen instructions to reset it back to the factory default.
BlackBerry: Remove the battery without turning off the phone and wait 30 seconds. Then reinstall the battery and turn the phone back on. Alternatively, you go to the Options screen, select Security, followed by Security Wipe. Check off all the options here and follow the onscreen instructions to reset the device to factory condition.
Windows Phone: From the Start screen (with the live tiles), tap the right-facing arrow and select Settings. Once inside, tap About, followed by Reset Your Phone. Tap Yes and follow the instructions.