Sorry to hear it and since I just opted for /e/ for the FP3 in our family, I unfortunately have no idea what could have gone wrong.
I am not so sure, if a general warning of the update is neccessary.
Annoying and frustrating as it is, things like that can always happen. (So a backup usually is a good idea (not that I would head my own advice all the time 
)).
I hope someone, who is more abled technically can help you out here.
There are some real 
wizzards 
here in this forum.
According to another thread the latest update seems to have worked for other users without any problems, if it was this update:
I.e. the latest one in this wiki