Here is a look at what to check when your iPhone won't turn on.
I'm Off and On Again
Instead of turning the iPhone off on a regular basis it tends to stay on indefinitely, and it just "goes to sleep" when it is not in use so it can save battery life. If you find that your iPhone is off without you actually turning it off yourself, or you find that your iPhone won't turn on, then there is something wrong. Here are a few ways to try and troubleshoot the issue when your iPhone won't turn on.
The most common reason that your iPhone won't turn on is also the most logical: it has no juice. If the iPhone's battery dies it is going to turn off on its own, and you are not going to be able to turn it back on. When you go through the motions of turning it back on by pressing the refresh and Home buttons it is going to give you the flashing image of an empty iPhone battery. Even when you plug it in to charge the battery, either to the computer or to a standard wall charger, the iPhone won't turn on right away. Instead, you just need to give it a few minutes of charge time and then you will see that the touch screen will return to the normal display of a battery charging. Once this happens you should be able to turn the iPhone on without problem, or it will turn back on itself depending on its original position. Try to reboot your iPhone several times.
Depending on what has happened the iPhone not turning on could have been because of how the battery died, what was happening when it died, or perhaps a series of applications or iOS problems during operation. In many of these cases the main point is that you are going to need to give it an essential shock to the system through syncing to your computer's iTunes. Unfortunately, this is not always easy to do as, if there was a problem then iTunes may not be able to recognize your iPhone and the iPhone still won't turn on. There are a few ways to try and get around this.
First, plug in your sync cable to your iPhone before you actually plug it into your computer. Open up iTunes on your computer, then hold the Home button down while you plug in the cable to your computer. From here you can choose to restore from your iTunes, which will restore your iPhone and will eventually allow it to turn on.
Another option is usually needed if you are using the iPhone while it has a very low battery and is still charging. The iPhone's battery can still die in this situation, and it can cause a connectivity issue and your iPhone won't turn on. You will likely not be able to just charge in the wall if this is the case, but try that first. If that fails then this is a clear indication that iTunes on your computer is not going to read it either. What you should do then is plug it in and restart your computer. Often this will allow the computer to read the iPhone and you can then try to sync or restore the iPhone while it is charging. You are going to have to give this process a little bit of time as it is not going to be immediate by any means.
The reality is that if the iPhone won't turn on after all of these attempts then it is likely going to be an outside issue that may have its own solution, or may not. The most common hardware cause of this is the Liquid Contact Indicators being activated. The Liquid Contact Indicators light up to signify that there has been significant liquid contact, which will likely result in water damage. This will essentially void the warranty and you will not be able to get a new free iPhone. What this will indicate to you is that your only real chance is to open up the iPhone and to try to dry it in the way that you normally do with an iPhone with water damage.
Physical trauma can also be a cause of an iPhone that won't turn on, and if it was dramatic enough to cause this then you will likely see some damage in the case or a cracked touch screen. Again, this is not something that is easily covered by your iPhone's warranty.
All other issues should just be taken to the Apple store, but make sure that you insist that no damage has been done to your iPhone.