Hi,
I'm sorry to hear that you are having problems with Notebooks! Do you remember the last thing you did in Notebooks?
A few suggestions:
1) open the multitasking bar and terminate Notebooks from there
2) if that does not help, go to your iPhone's settings > Notebooks and select "Start at Top"
3) If this still does not fix the issue, please send the crash logs which are created whenever an app crashes to notebooksapp@me.com. This is where you find the logs:
When you synchronize with iTunes, all the crash logs are transferred to your computer. Here are their locations:
• Mac OS X :~/Library/Logs/CrashReporter/MobileDevice/
• Windows XP: C:\Documents and Settings\\Application Data\Apple computer\Logs\CrashReporter/
• Windows Vista: C:\Users\\AppData\Roaming\Apple computer\Logs\CrashReporter/MobileDevice/
The log file names start with Notebooks and have the extension “crash”. They are just plain text files and can be sent by e-mail in original or zipped form, or even copy-pasted into your e-mail program.
(By the way: you do not need to worry about your documents, they are unharmed; as it seems, something during the startup procedure keeps Notebooks from launching. In the worst case you can still export and access all your documents through iTunes' file sharing pane.)
I hope that helps.
Best,
Alfons