If you are unable to find your patient's clinical note, the most likely scenario is that associated appointment was deleted and therefore the clinical note attached to that appointment was also deleted.
To verify that this was the cause follow these steps:
1. Go to Clinical > Audit Log
2. Search for the Patient Name. Make sure that the Date Range is correct and select Update Filter.
3. If the appointment was deleted there will be a log of the deleted appointment
If you need to recover the appointment, learn how to here.