Abigail 
I have checked the audit log
- nothing in there 
I have used incognito mode
- notes are not there

I have cleared the cache

- notes are not there.
If you go to customers, and pick one, normally the notes are at the top in the grey area - they aren't
If you click edit for the customer, you go to customer details, there is normally a tab for notes - that isn't there.
it has just disappeared along with the notes.