How could I go back to the beginning of the message in iMessage ?

I don't know if there is some limit on the length of a messaging conversation that is visible, but I assume that you cannot swipe all the way up to the top for this message to see the beginning of it?

All that I can suggest is to quit Messages and reboot and see if the entire message appears.

Go to the home screen first by tapping the home button. Double tap the home button and the recents tray will appear with all of your recent apps displayed at the bottom. Tap and hold down on any app icon until it begins to wiggle. Tap the minus ➖ sign in the upper left corner of the app that you want to close. Tap the home button or anywhere above the task bar.

Reboot the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.

Describe what is happening. Can you swipe at all in this message?

Quit messages, reboot and then see of the message can be loaded completely.

In order to quit the Messages app, Go to the home screen first by tapping the home button. Double tap the home button and the recents tray will appear with all of your recent apps displayed at the bottom. Tap and hold down on any app icon until it begins to wiggle. Tap the minus ➖ sign in the upper left corner of the Messages app. Tap the home button or anywhere above the task bar.

Reboot the iPad by holding down on the sleep and home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider if it appears on the screen - let go of the buttons. Let the iPad start up.

If that doesn't work, the only other thing that I can think of is to restore from a backup - but only if you have a backup that contains that message. When you restore from a backup, you restore everything in that backup onto the iPad, so if your backup is one week old, you will turn back the clock one week with regard to everything else on the iPad and that would erase any current messages on the device.

How to backup and restore from a backup

http://support.apple.com/kb/HT1766