It is possible that your PAF software has been damaged by a power glitch or something similar. Ubder normal circumstances I would suggest that you remove it and install a fresh copy from familysearch.org but the download link for PAF has not been working for the past two days.
In the meantime try renaming and unzipping a backup to convert it back into a .paf file. Will it open successfully? If so, the problem is probably the Restore function in PAF. If it will not open, the problem is more likely in the Backup function. (Note that renaming is important because restoring a file to the same and location as another file will cause Windows to delete the existing file and replace it with the one created by restoring.)
Take your backup to a different computer running PAF. Can that computer Restore the backup successfully?
There are other ways of backing up your data. One is to export everything as a gedcom file. The other is to use the Save As command to save the file under a different name.