Hello, there is a long thread in the Apple Support Communities
HERE about losing notes in the Notes App; seems that some have traced the issue to a setting for Notes in their e-mail accounts, so you might want to reviewed the thread and check out your e-mail settings - seems like Apple support was pretty worthless in figuring out this issue.
If not helpful, I'd probably 'Restart' or if not successful 'Reset' my iPad and determine if that helps - also, I've not checked my sync settings for Notes on my computer and not sure if notes can be lost in a sync from the iPad and then restored (like w/ music & photos)? If none of these suggestions help, please post back - this seems to be not an uncommon issue that Apple needs to address - good luck!
P.S. I put
Evernote on my iPad but have not even used it yet - probably will switch over my note taking immediately after reading that thread -