Step 1 – Leave it on Charging
Check for the battery. Recharge for a couple of hours and see if it restarts.
Step 2 – Check Your Charger and Data Cable
Connect your iPhone/ iPod/ iPad to the Computer if it charges fine then its time order a new charger also if you have a another iPhone/ iPod/ iPad/ iPod/ iPad which are working fine try the cable on those devices or their cable on your iPhone/ iPod/ iPad. If this doesn’t work then proceed to next step.
Step 3 – Try a Different USB Port
If that doesn’t work, check if USB port that is being used to connect to your PC or MAC is functioning properly. Connect and try charging and connecting to a different port. Your iPhone might not actually be recharging due to a faulty USB point.
Step 4 – Hard Reset
Press and hold the home button and the power button together until the silver apple logo is shown on screen [First the "Slide to Power Off" message will appear and then the screen will go blank and then the apple logo will appear]. Once the apple logo appears you can release the buttons, If this doesn’t work then proceed to next step.
Step 5 Manual Reset
If booting still fails, try to reset your iPhone. You can reset your iPhone by pressing and holding the Sleep/Wake button and the Home button simultaneously for a few seconds, to see if booting begins.
Step 6 – The Mute Switch Method
While Connected to a power source turn on and off your mute switch 4-5 times and then hold the power and the Home button for10 seconds and then release, Apple Logo should Appear. (like you did in the hard reset method way)
Step 7-Put it in DFU mode and Restore
If the problem persists and the phone still does not start, put the iPhone into DFU mode and attempt to restore from there. When iTunes now identifies the iPhone try to restore to an official firmware. (Caution if you have upgraded your phone using custom firmware to unlock your iPhone then you can’t restore the phone with it again, you will have to restore it with the original Firmware which will upgrade your baseband and lose your unlock Hence first you will need to downgrade your baseband. iPhone 4 users can only downgrade if they have saved the SHSH Blobs or iPhone 4 user can upgrade to iOS 4.3.3 and use GeveySim / Rebel Sim to unlock, iPhone 3G and 3GS users only need to downgrade if they are using iOS 4.1 and above click here to Downgrade to iOS 3.1.3)
Step8- Call an Expert
Still not working ? There might be some hardware complication which would need technical expertise. Call up your nearest Apple helpline number or your vendor for support.