Click here to see a HowToGeek article called "What to Do When Your Android Phone Won't Turn On"
It is possible that something can go totally wrong with your smartphone rendering it practicably useless - however it can be put back to 'as new' condition by performing a factory reset as described in this guide.
However a factory reset of your smartphone will 'wipe it' by restoring the Android operating system to it's original Factory Settings and all of your personal data and some tweaked system settings will be lost.
So before doing a factory reset you should ensure your pictures, contacts, songs etc. are all backed up so that you can re-instate them later. The best advice here is to arrange for everything to be backed up to your Google account then recovering the data is relatively straight forward.
Click here to see a short video clip that demontrates the stages and what buttons to press.
An example of some system tweaks that you may have applied to your phone is this useful one to do with smooth screen performance.
As a reminder these are the necessary steps to follow to obtain 'Developer Options' and change settings ......
Developer Options
Open up your system settings and tap "About phone." (If you don't see that in the main settings menu, try looking in the System section.)
Scroll down 'til you see a line labeled "Build number," and — silly as it may sound — tap that line seven times. (On some phones, you may have to tap "Software information" first to find the "Build number" line.)
Your phone will confirm that you want to activate Android's developer mode and probably ask you to put in your PIN or pattern to prove that it's really you. Do it!
Once that's done, back out to the main settings menu and look for a new section labelled "Developer options" at the bottom. If you don't see it there, look in the System section and tap "Advanced" to find it.
Remember this is an advanced section of Android's settings and is not something an average user should generally mess around with. Unless you want to risk really messing things up, adjust only the specific areas mentioned here and leave everything else alone.
Changing Settings
In the new "Developer options" area, scroll down until you see the "Drawing" header. There, you should find three animation-related settings: "Window animation scale," "Transition animation scale," and "Animator duration scale."
Tap each of those and change its value to "Animation scale .5x" (and make sure you're selecting ".5x" — with a decimal).
Back out to your home screen and try moving around your phone — opening the app drawer, opening an app and then returning to the home screen, and so on. Things should seem noticeably snappier.
If you think there's room for further improvement, you can try heading back to that same menu and changing those settings to "Animation off." It's a more extreme jump — and for most people, the ".5x" setting will be the optimal middle-ground — but if your phone is seriously lagging or you just feel the need for even more speed, it's an option worth exploring.