Kathye- If I were to make a wild guess here, I would speculate that there have been some changes made to the Notes field setup in PAF 5.2.18, and that you managed to open your file (once) in the newer 5.2.18 version, and then reverted back to the 5.1.12 version.
Suggest that you first make a backup of whatever data file you now have and then verify you are in version PAF 5.2.18. Then manually open your file (wherever it may now reside) from the newest version of the program. If your notes reappear, stop. Find someone local to help you sort it out.