If your journal suddenly appears to be empty, the issue may be with iCloud or with the device. Try force-quitting and restarting the app, turning the device completely off and on again, and turning iCloud off and on again, as described in the FAQ entry for “The app stopped working correctly" below.
You may also get your journal back by restoring your device to its most recent backup. Please see this Apple support article for detailed instructions on how to back up and restore your device's content, including the Journal:
iOS: How to back up and restore your content (http://support.apple.com/kb/HT1766).