It may be a corruption of the program or the data. If it is feasible, copy your file to a flash drive and try it on someone else's PAF program (perhaps at a Family History Center).
You might also try this at home: Make a fresh backup with a new name (include today's date in the file name, so you can easily identify the new backup).
Then run File menu > Check/Repair 3 or 4 times (less if no errors are reported).
Then restart your computer to see if the problem is solved. On the Individual screen there is a scroll bar that will allow you to scroll up and down the list. By default, you will be shown the part of the list showing the main person. Clicking the Full Name heading will reverse the current sort order. But in any case, you should be able to scroll up and down the list from start to finish.
If Check/Repair mangles your data (a remote possibility), restore the backup to return to where you were when the backup was made. Then export ALL your data as a gedcom file, create a new PAF data file with a new name, and import the gedcom. That will usually solve all problems.
If the problem persists, use the Windows Control Panel to uninstall/remove PAF. Then download a fresh copy of PAF from www.familysearch.org and install it. Run it to open your database.
So there you have my Plan A, Plan B, Plan C, and Plan D. Use as much as you need.