Unfortunately it ever so often happens that Prime file get corrupted in some way and refuse to load. It seems to happen more often with larger files containing embedded pictures.
Prime does not provide any means for repairing such files nor any means like auto saving a backup copy to cope with that problem. So you are on your own thinking on making backups on a regular basis.
So the best option would be to fall back to your latest backup file.
Of course you may post the worksheet here and maybe someone in the forum might be able to repair at least parts of it ... no guarantee.
