First of all, are you sure it is charged? Have you tried using it while plugged in to the wall charger?
Letting a Lithium battery sit completely unused for long periods of time is not good and can even ruin the battery. If the battery drained to the point of auto-shutdown, and then sat idle for months and eventually completely drained, it may never take a charge and work properly again.
If you plug it in and it will not even power on, then I think you have a hardware failure and will need to take it in to Apple to get the battery replaced.
If it seems to power on and tries to start up, you can try a reset, or if that does nothing, a restore. See the procedures in this article under "restart" and "restore" - http://www.apple.com/support/iphone/assistant/phone/
Dear Michel Black,
Thank you for your response.
I faced no problem in re-charging the battery. I followed all the procedures mentioned in the link referred by you.
But the problem is persisting. The touchscreen display shows " slide to unlock". But, the touch screen is not responding and hence I am unable to unlock the iphone. The touchscreen is not responding at all.
You can try a reset and/or a restore
see instructions at http://www.apple.com/support/iphone/assistant/phone/
If neither makes a difference, then I'd say it is time to take it into Apple. If it is still under the one year warranty, you may be able to just exchange it for a factory refurbished unit. Otherwise, you may still be able to do that, but for a fee under the out of warranty exchange program.