The iPhone displays the charge amount left in the battery on the top-right corner of the screen. When your battery is very low, charge it to prevent it from going completely dead. Once your iPhone battery is completely dead, it is difficult to tell if your problem is simply a dead battery or something more serious. You'll need to plug your iPhone into an outlet or your computer and begin the charging process to know if your problem is a dead battery.
Plug your iPhone into an outlet using the Dock Connector to USB cable and USB power adaptor, or to your computer using the Dock Connector to USB cable.
Press the "Home" button at the bottom of your iPhone to see the battery status if it does not automatically pop up. If the power is very low, you will see an image of an empty battery with a small red bar to the left. A lightening bolt will appear under the battery image to alert you that you need to wait a few moments to use your phone while it gathers a charge.
Wait a few minutes if no image appears when you press the "Home" button the first time. If the battery is completely dead, you will need to wait a few minutes before the iPhone has enough power to display the low battery warning.