If you were syncing your Notes with an account and recently removed that account, or turned off Note syncing in that account, you can probably recover them by simply turning that account and syncing back on. They are usually stored in a special Notes folder in that account's email.
If you noticed them there and deleted them, look in your email trash and put them back.
Also, if you have more than one account set up to sync with Notes, or have both local and synced notes, there will be an Accounts button above your list of notes. Tap it and make sure you are not just looking at an empty account.
If the Notes app really did just dump things:
First try a restart or reset. This has been known to recover lost data in apps before (though I don't remember it ever working specifically for Notes)
If you've been doing your backups, either with iTunes or iCloud, and don't mind loosing what else you've done since the last backup, a restore from backup may give you back your notes.
As a note taking app, I prefer Evernote. It syncs online an on your computer (if you install it there). It is far more versatile than Notes, and as long as you're not a rabid or professional note taker, the free account will be all you ever need.
That's all I can think of at the moment.
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When you have problems with an app it's a good idea to clear it from RAM.
Close/Clear the app: With the problem app closed, double tap on the Home button to get the multitask bar. Tap and hold on any icon in the bar until they jiggle, then tap the minus sign on the problem app. This will remove it from RAM and insure a clean start the next time you launch the app.
If that does not work try a restart,
Restart: Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears.
and then a reset if that does not work.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.
