Let's assume that somehow your battery was completely drained and the iPad shut itself off. If this is the case plug it into the charger that came with it and plug that into a good wall outlet. Wait at least 30 minutes. An iPad with a completely drained battery often will not respond for a while until the battery get a certain amount of charge back. After 30 minutes on the charger reset the iPad. Press and hold both the Home button and the Sleep/Wake button continuously until the Apple logo appears. Then release the button and let the device restart. You will not lose data doing this. It's like a computer reboot.
If the iPad restarts let it continue to charge until it is at 100%.
If the iPad doesn't restart follow the links provided by jason269 and see if the iPad eventually responds.
If none of the above helps it's time o have your iPad looked at by an Apple technician. Make an appointment at an Apple Store to have your device examined by a technician. Or contact Apple Support. Be sure to back up your device before taking or sending it to Apple.
Also, I advise that you begin a regular backup program to same your "memories" that you don't want to lose. Every type of computer device will eventually die and having your data safe requires a good backup strategy.
