With PAF, a little routine maintence can prevent some major headaches such as corrupted or lost databases.
Backups Before doing any sort of over-haul of your PAF database, take 5 minutes to do a backup. Then if the merge or import goes awry, it only takes a few seconds to restore your database to its previous state. If backups have unique names, you can restore them safely without inadvertently replacing a database you want to keep. (An easy way to create a unique name is to include the date in the name of the backup file.)
Check/Repair This little item under the File menu can fix many simple problems before they destroy your database. If it reports that it is unable to repair your database, run Check/Repair a few more times to see if it can fix the problem. If that fails to solve the problem, often the problem can be fixed by exporting All the database file to a gedcom file. Then create a New file in PAF and Import the gecom file into it. Again remember to use a unique name, or PAF may replace an existing database with blank one. If you don't have a good backup and PAF is unable to repair your database, perhaps PAF Insight (from ohanasoftware.com) may be able to repair it. Many Family History Centers have it.
Keep a current backup on some form of removeable media: (flash drive, floppy, CD/DVD) That way when your hard drive self-destructs you don't lose your hard work. To be extra safe, you might keep a few backups somewhere other than in your home. (e.g. at the home of a friend or relative, or online) I have seen some bad experiences with floppies, but if they can't be read by one computer, don't format them, try reading them on a different computer. Often this will solve the problem.