Resetting the iPhone can be done in 2 ways, either a soft reset or a hard reset.
1. On your iPhone 4 press and hold both the Sleep/Wake button and the Home button for about ten seconds. After ten seconds you should see the Apple logo showing up.
2. Now try to run you phone again and see if it works.
1. Plug in the device to the computer and start iTunes.
2. Press and hold the Sleep/Wake + Home for 10 seconds.
3. Once you see the Apple Logo appears, release the power and continue to hold down the home button for about 5-10 seconds. Look in iTunes and see if your device have shows up.
4. iTunes will notify you that you are now in Recovery mode. Click on OK to continue with the restore process.
5. Next press and hold on the SHIFT button and click on the Restore button. Select the latest IPSW file and the prompt appears.
6. iTunes should now attempt to get your device back to the manufacturer setting.
7. iTunes will later give you an opportunity to restore you content from the PC. Every time when you do a sync, iTunes makes a backup so you can choose the latest backup from the sync.