In this incidence I have no idea why it happened. You need to check Win and QV logs. Have had incidences where the .pgo files becomes corrupt resulting the same as you. Have solved it with backing those files up every night so they easily can be transferred back when needed.
I have this happen before when the pgo files have become corrupted, but could never pinpoint the root cause of it happening i'm afraid, as Michael says good to take a backup of these on a regular basis
One possibility to check if there are (unexpected) changes within the pgo-files could be to load these pgo-files with a timestamp into a qvw. Within the server settings there is an option pgoasxml which created the pgo-files additionally as xml-files.